I'm not sure if this is an issue with KMix or ALSA / PulseAudio in general, but it existed since I moved to Linux. My overall sound volume appears to be incorrectly scaled, and 55% volume acts like 0% (muted). I can however set the volume slider anywhere between 0% and 100%. Anyone know how to fix this and properly scale audio volume?

To eliminate Pulseaudio as the cause, can you try temporarily disabling it (or otherwise bypassing it, by using an application which allows you to select your preferred ALSA device directly, such as aplay) to see if that has any impact on the volume scaling?

This is quite likely however to be an ALSA level issue - what is the lspci entry for your audio chipset?

Tried running Audacious through aoss and padsp, and directly playing a wav file through aplay and paplay. The issue persists in all cases.

As for lspci, those are my devices. I use wireless headphones from Logitech, connected via a nano-USB controller.

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00:00.0 Host bridge: Intel Corporation 5520/5500/X58 I/O Hub to ESI Port (rev 13)
00:01.0 PCI bridge: Intel Corporation 5520/5500/X58 I/O Hub PCI Express Root Port 1 (rev 13)
00:02.0 PCI bridge: Intel Corporation 5520/5500/X58 I/O Hub PCI Express Root Port 2 (rev 13)
00:03.0 PCI bridge: Intel Corporation 5520/5500/X58 I/O Hub PCI Express Root Port 3 (rev 13)
00:10.0 PIC: Intel Corporation 7500/5520/5500/X58 Physical and Link Layer Registers Port 0 (rev 13)
00:10.1 PIC: Intel Corporation 7500/5520/5500/X58 Routing and Protocol Layer Registers Port 0 (rev 13)
00:11.0 PIC: Intel Corporation 7500/5520/5500 Physical and Link Layer Registers Port 1 (rev 13)
00:11.1 PIC: Intel Corporation 7500/5520/5500 Routing & Protocol Layer Register Port 1 (rev 13)
00:13.0 PIC: Intel Corporation 7500/5520/5500/X58 I/O Hub I/OxAPIC Interrupt Controller (rev 13)
00:14.0 PIC: Intel Corporation 7500/5520/5500/X58 I/O Hub System Management Registers (rev 13)
00:14.1 PIC: Intel Corporation 7500/5520/5500/X58 I/O Hub GPIO and Scratch Pad Registers (rev 13)
00:14.2 PIC: Intel Corporation 7500/5520/5500/X58 I/O Hub Control Status and RAS Registers (rev 13)
00:15.0 PIC: Intel Corporation 7500/5520/5500/X58 Trusted Execution Technology Registers (rev 13)
00:1a.0 USB controller: Intel Corporation 82801JI (ICH10 Family) USB UHCI Controller #4
00:1a.1 USB controller: Intel Corporation 82801JI (ICH10 Family) USB UHCI Controller #5
00:1a.2 USB controller: Intel Corporation 82801JI (ICH10 Family) USB UHCI Controller #6
00:1a.7 USB controller: Intel Corporation 82801JI (ICH10 Family) USB2 EHCI Controller #2
00:1b.0 Audio device: Intel Corporation 82801JI (ICH10 Family) HD Audio Controller
00:1c.0 PCI bridge: Intel Corporation 82801JI (ICH10 Family) PCI Express Root Port 1
00:1c.1 PCI bridge: Intel Corporation 82801JI (ICH10 Family) PCI Express Port 2
00:1c.4 PCI bridge: Intel Corporation 82801JI (ICH10 Family) PCI Express Root Port 5
00:1c.5 PCI bridge: Intel Corporation 82801JI (ICH10 Family) PCI Express Root Port 6
00:1d.0 USB controller: Intel Corporation 82801JI (ICH10 Family) USB UHCI Controller #1
00:1d.1 USB controller: Intel Corporation 82801JI (ICH10 Family) USB UHCI Controller #2
00:1d.2 USB controller: Intel Corporation 82801JI (ICH10 Family) USB UHCI Controller #3
00:1d.7 USB controller: Intel Corporation 82801JI (ICH10 Family) USB2 EHCI Controller #1
00:1e.0 PCI bridge: Intel Corporation 82801 PCI Bridge (rev 90)
00:1f.0 ISA bridge: Intel Corporation 82801JIR (ICH10R) LPC Interface Controller
00:1f.2 SATA controller: Intel Corporation 82801JI (ICH10 Family) SATA AHCI Controller
00:1f.3 SMBus: Intel Corporation 82801JI (ICH10 Family) SMBus Controller
01:00.0 SATA controller: Marvell Technology Group Ltd. 88SE9128 PCIe SATA 6 Gb/s RAID controller (rev 11)
02:00.0 USB controller: NEC Corporation uPD720200 USB 3.0 Host Controller (rev 03)
03:00.0 VGA compatible controller: Advanced Micro Devices [AMD] nee ATI Barts XT [Radeon HD 6800 Series]
03:00.1 Audio device: Advanced Micro Devices [AMD] nee ATI Barts HDMI Audio [Radeon HD 6800 Series]
05:00.0 SATA controller: JMicron Technology Corp. JMB363 SATA/IDE Controller (rev 02)
05:00.1 IDE interface: JMicron Technology Corp. JMB363 SATA/IDE Controller (rev 02)                                                 
06:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168 PCI Express Gigabit Ethernet controller (rev 03)         
07:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168 PCI Express Gigabit Ethernet controller (rev 03)         
08:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL-8185 IEEE 802.11a/b/g Wireless LAN Controller (rev 20)             
08:06.0 FireWire (IEEE 1394): Texas Instruments TSB43AB23 IEEE-1394a-2000 Controller (PHY/Link)
3f:00.0 Host bridge: Intel Corporation Xeon 5500/Core i7 QuickPath Architecture Generic Non-Core Registers (rev 05)
3f:00.1 Host bridge: Intel Corporation Xeon 5500/Core i7 QuickPath Architecture System Address Decoder (rev 05)
3f:02.0 Host bridge: Intel Corporation Xeon 5500/Core i7 QPI Link 0 (rev 05)
3f:02.1 Host bridge: Intel Corporation Xeon 5500/Core i7 QPI Physical 0 (rev 05)
3f:03.0 Host bridge: Intel Corporation Xeon 5500/Core i7 Integrated Memory Controller (rev 05)
3f:03.1 Host bridge: Intel Corporation Xeon 5500/Core i7 Integrated Memory Controller Target Address Decoder (rev 05)
3f:03.4 Host bridge: Intel Corporation Xeon 5500/Core i7 Integrated Memory Controller Test Registers (rev 05)
3f:04.0 Host bridge: Intel Corporation Xeon 5500/Core i7 Integrated Memory Controller Channel 0 Control Registers (rev 05)
3f:04.1 Host bridge: Intel Corporation Xeon 5500/Core i7 Integrated Memory Controller Channel 0 Address Registers (rev 05)
3f:04.2 Host bridge: Intel Corporation Xeon 5500/Core i7 Integrated Memory Controller Channel 0 Rank Registers (rev 05)
3f:04.3 Host bridge: Intel Corporation Xeon 5500/Core i7 Integrated Memory Controller Channel 0 Thermal Control Registers (rev 05)
3f:05.0 Host bridge: Intel Corporation Xeon 5500/Core i7 Integrated Memory Controller Channel 1 Control Registers (rev 05)
3f:05.1 Host bridge: Intel Corporation Xeon 5500/Core i7 Integrated Memory Controller Channel 1 Address Registers (rev 05)
3f:05.2 Host bridge: Intel Corporation Xeon 5500/Core i7 Integrated Memory Controller Channel 1 Rank Registers (rev 05)
3f:05.3 Host bridge: Intel Corporation Xeon 5500/Core i7 Integrated Memory Controller Channel 1 Thermal Control Registers (rev 05)
3f:06.0 Host bridge: Intel Corporation Xeon 5500/Core i7 Integrated Memory Controller Channel 2 Control Registers (rev 05)
3f:06.1 Host bridge: Intel Corporation Xeon 5500/Core i7 Integrated Memory Controller Channel 2 Address Registers (rev 05)
3f:06.2 Host bridge: Intel Corporation Xeon 5500/Core i7 Integrated Memory Controller Channel 2 Rank Registers (rev 05)
3f:06.3 Host bridge: Intel Corporation Xeon 5500/Core i7 Integrated Memory Controller Channel 2 Thermal Control Registers (rev 05)

As the sound card you are using is USB based, we'll need the output of lsusb here. I am guessing the regular sound card scales the volume correctly?

Back in Windows the volume was scaled fine. The problem started happening when I switched to Linux / KDE.

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mircea@linux-qz0r:~> lsusb
Bus 003 Device 007: ID 05e3:0606 Genesys Logic, Inc. USB 2.0 Hub / D-Link DUB-H4 USB 2.0 Hub
Bus 004 Device 002: ID 045e:076d Microsoft Corp. LifeCam HD-5000
Bus 005 Device 002: ID 1131:1004 Integrated System Solution Corp. Bluetooth Device
Bus 006 Device 002: ID 046d:c52b Logitech, Inc. Unifying Receiver
Bus 006 Device 003: ID 046d:0a29 Logitech, Inc.
Bus 009 Device 002: ID 056a:00d1 Wacom Co., Ltd Bamboo Pen & Touch (CTH-460-DE)
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 006 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 007 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 008 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 009 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 003 Device 008: ID 03f0:5711 Hewlett-Packard PhotoSmart C4100 series
Bus 003 Device 009: ID 05e3:0606 Genesys Logic, Inc. USB 2.0 Hub / D-Link DUB-H4 USB 2.0 Hub
Bus 010 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
mircea@linux-qz0r:~>

Does the regular sound card (ie. the non-Logitech, integrated one) scale volume correctly?