I also use gentoo, KDE, and alsa, not installed system wide, using the kernel driver, but the separate driver should be fine... and sound has always worked fine for me. You don't want to use 'aoss' that is for old programs that only have OSS support. Just run planeshift normally, and you could try running './pssetup' and see if sound is disabled there.
The xml file (/art/soundlib.xml) uses Crystal Space's VFS (virtual file system), don't worry about the lowercase 'planeshift' at the start of all paths, it is a link to where ever it's installed.
If none of that helps, try opening the 'psclient.cfg' file, and see if you can find something similar to what is in mine:
;Sound renderer
System.PlugIns.iSndSysRenderer = crystalspace.sndsys.renderer.software
; Sound Loader
System.PlugIns.iSndSysLoader = crystalspace.sndsys.element.loader
; Linux - ALSA buffer sound setting
SndSys.Driver.ALSA.SoundBufferms = 150
I don't really know off hand how it's enabled or disabled in that file, I normally just use pssetup.