I am having a hell of a time to get my headphones (Logitech Comfort Clearchat USB) to work in KDE. They work just fine in Windows and Cinnamon so I know that they're just fine. In KDE however I set them as prefered device in Phonon. So, as long as I have them plugged in, Amarok will output to my headphones. However nothing else does. I did mess around with the audio settings in umplayer and now it will output to my headphones, but nothing I did to VLC would and Firefox simply refuses to output to anything other than my speakers. I'm not even sure where to begin trying to get Firefox to cooperate. I installed Pulseaudio to see if it would make a difference but it hasn't. Anyone have any ideas? I'm at my wit's end.

Sadly you failed to tell us any version of the installation you have, e.g. KDE version, distribution and version, Phonon version and backend.
Then please make sure nothing is muted in Pulseaudio nor KMix

FWIW: not all applications use Phonon, Firefox and VLC for example don't.

Ah, that I did. I'm using KDE 4.8.3. I'm using Phonon version 1:4.6.0-2. I've already checked any everything is unmuted.

What do Firefox and VLC use? If I could discover that I might be able to solve my problem.

Still, no information about the distribution? Some use Pulseaudio by default, in that case you should use Pulseaudio for all configurations regarding sound hardware, e.g do not modify the Phonon settings.

Of course pulseaudio should be compiled with phonon support enabled which allows you to do all hardware configuration through pulseaudio. So information about the distribution you are using is quite useful. Please set it also in your profile so we don't have to ask

Ah I see, I apologize for leaving out pertinent information, I'm running Arch Linux. It had ALSA by default.

Edit: At this point I have no idea about anything. I haven't messed with my audio since this morning, all I did was install and look around pavucontrol, though nothing I did in there helped. I gave up for now, a few hours passed, and now suddenly things have changed without me doing a thing. Now when I put my headphones in sound will simply become very quite, but will still be output through my speakers. I can't even hear things in Amarok, whether I've got my headphones plugged in or not.

Now every time I open up Phonon, sometimes even Amarok I get the message "KDE detected that one or more of these internal devices were removed." It then asks me if I want KDE to forget about them. Sometimes it only lists "Output:HDA Intel, ALC888 Analog (Default audio device)", sometimes it lists that plus "Built-in Audio Analog Stereo".

Edit: Well I set Built-in Audio Analog Stereo above RV770 HDMI Audio in Phonon, so how I can hear sounds from Amarok and Umplayer again, so when I plug my headphones in audio output is directed to them. Unfortunately I still can't get Firefox or VLC to do the same. I'm not as concerned about VLC, what I'm really wanting to play nice is Firefox.

Getting closer... VLC will now output to my headphones. All I did today was choose my headphones as the master channel in Kmix, which solved my issue of not being able to control headphone volume with my keyboard. Now the only application that continues to refuse to work with me is Firefox. Chromium will output audio to my headphones so I know that at least some browsers are cooperating.

Alsa is default on almost all distributions, the alternative would be OSS. The question was more if it had Pulseaudio by default or not, as this is an additional layer to handle audio on top of Alsa. If you use Pulseaudio then you shouldn't change anything in either Alsa, Kmix or Phonon at all, just leave it default and use Pulseaudio for configuring output devices. Sadly I don't know about Arch's default, though.

Like I said, Arch had ALSA by default, and through messing with Phonon, Kmix, and some dark arts I've managed to get everything but Firefox to cooperate. I've detailed everything I've done in previous posts.