I see this info on on the wiki.
Source: http://wiibrew.org/wiki/MPlayerWii
ÂÂÂÂ* Containers: AVI, WMV, MKV, OGM, MP3, FLV (Flash Video), MP4, M4V, QuickTime, 3GP
ÂÂÂÂ* Video codecs: Xvid, DivX, Windows Media, MPEG 1/2, H.264
ÂÂÂÂ* Audio codecs: MP1/2/3, Windows Media, AAC
Any other...