Before encoding, the loop plays back smoothly; after, there seems to be a half-second gap at the end.
The command I'm using is:
lame -q 0 -V 8 -b 32 -B 320 --resample 44.1 -m j music.wav music.mp3
I've tried all sorts of variations of the above. The output file must be 44100hz so that Adobe Flash will accept it for import.
WAV and MP3 files: