I have played around the sound features of MMAPIs in S60 SDK Beta 0.3, and observed some problems. It looks that the MMAPI is not powerful enough to decode all kinds of .wav and .mid data. I tried some sample sound data such as laser.wav and song.mid from Nokia's sample package, my midlet can play back them using MMAPI without any problem. But when I attempted to use the midlet to play some .wav and .mid I downloaded from some popular web sites, the emulator cannot handle the format and throw exceptions. Have you seen such problems?
I'm afraid I haven't tested out other than standard midi-polyphonic-ringtone files, and they have of course been thouroghly tested as ringtones.
It could be a size-question, but I'm afraid I have to test it a bit more before I know.
I'll get back to you if I find out anything. I have also tested out the "Sony Ericsson T610" which support MIDP 2.0, so you could download that and see if the same thing happens there...