What configuration file stores the volume percent amount that multimedia keys (like XF86AudioRaiseVolume) change? The current setting of 11% is too high for me.

As far as I know the amount can't be changed, but is 5% on my system. Please try removing the KMix configuration, stored in the files kmixctrlrc and kmixrc. In addition, profiles are stored in ~/.kde4/share/apps/kmix which can also be removed.

These three removals should be done with KMix closed (including not being present in the system tray)

I killed kmix and removed /root/.kde/share/config/kmixctrlrc and /root/.kde/share/config/kmixrc. There wasn't a .kde4 directory (this is KDE3) and there wasn't a kmix directory under /root/.kde/share/apps

But when I press a volume button, kmix is automatically restarted and appears in the tray, and the volume amount is still 11%.

Somebody told me that for gnome, the answer would be to execute the following command:
"gconftool-2 -s /apps/gnome_settings_daemon/volume_step --type int 2"

But they said the PATH to volume_step would be different for KDE, and I don't know what it is.

If you have components of the GNOME framework running at the same time as KMix, then they could both be increasing the volume. Can you ensure that GNOME daemons aren't running alongside KMix when you change the volume by keypress?