Well I installed sonicstage (bleh! what a disgustingly bloated program. not to mention all the installers come with their own little downloaders that they like to install onto your computer...
http://www.sony-asia.com/support/download/60444 says sup). Found sonywavwriter.ax, and i can upload it here if anybody wants to skip the trouble (is there any rule against me attaching it on these forums?). Can't find where the temporary files for the installer were downloaded though so I'm still looking for the OpenMG installer piece.
As knws mentioned in the thread you linked me to, uninstalling sonicstage actually does not uninstall openMG (god no uninstaller can be trusted these days. gotta switch to a *nix system and rock out with a package manager or something I can actually rely on), which is pretty gross. I should have uninstalled this stuff with revo. Anyway I will try jcpsp at this point and see what happens
edit: aaaand sound seems to work! i'll go through the process I followed step by step:
1) installed sonicstage (bleh.)
2) searched for sonywavwriter.ax (that is the exact spelling). copied this file somewhere else
3) uninstalled sonicstage (kill it with fire!)
4) used regsvr32 to re-register sonywavwriter.ax
5) booted up jcpsp and loaded an iso of trails in the sky; I can hear sound during sound-causing events so that seems to be all there
since I didn't uninstall OpenMG, that stuff was still lying around and the codec was still registered, so I only had to register wavwriter to make it go. I will continue experimenting and see what happens, I guess