Is there an MP3 player?
Currently we use Intel's highly optimised library for the ARM processor that includes MP3 decoding support. The MP3 player in podzilla that uses this library runs quite well, but it isn't perfect - some songs will skip, VBR isn't handled too well and high bitrates aren't always decoded fast enough. With current developments on 4G iPods, realtime playback is possible and these problems generally don't apply.
The MAD (
http://www.mars.org/home/rob/proj/mpeg/) player also runs, however it is not quite real-time.
The Helix MP3 Decoder (
http://datatype.helixcommunity.org/mp3dec) has been tested and runs almost as fast as the Intel library.
[edit]
Is there an Ogg player?
The Tremor (
http://www.xiph.org/ogg/vorbis/) player is running at about 80% real-time. With current developments on 4G iPods, realtime playback is possible.
[edit]
What about FLAC, AAC, Mod, and wave files?
Right now some development is happening on MPD which supports these formats as well MP3 and OGG Vorbis.
An AAC player is in the current beta of podzilla. See the blog entry (
http://www.ipodlinux.org/blog/index.php?p=15) for details.