Looks like I broke getLength() and getOffset() for mp3 fileplayers in 1.10. 
And I think the problem with isPlaying() and the finish/loop callbacks is related--because length and offset aren't getting computed correctly it never sees that it's reached the end of the file. I've pushed a fix for that, hopefully we'll be able to get that out in the next update.
Unfortunately, we don't support WAV playback directly. The compiler converts WAV files to .pda format which is similar but slightly different. I looked into doing native WAV support but there were a bunch of audio formats in WAV (e.g. 24-bit, ADPCM..) that would be difficult to handle. But maybe we could do 16-bit PCM, since it's identical to .pda after the header. I'll file that now.