I'm still working out the details on how audio actually works. I've currently got SPX, AAC, WAV, and FLAC encoding options set up, and they all seem to be working. I dunno if MP3 will work, there are patent encumbrances on it; but there might be a Nokia MP3 plugin available... Is "OGG" actually an audio encoding? I thought it was video. Let me do some research.

EDIT: Ah, yeah, "OGG" is the mechanism used for packaging audio and video together. (Kind of like Matroska.) There are several open source audio formats; I've already got support for Speex (an encoding format optimized for voice data). There's also Vorbis and Opus, which are higher-end encodings for music. Dunno just how much of a load using those would place on the N900, though.