if you're trying to export a flash animation with createjs, you might get this error:
Assertion failed: (inUsed != 0), function l3encEncode, ../mp3encoder/wrappers/enc/l3/encifc.c, line 591
This sometimes happens when you use a .wav file in your timeline and library to adjust the bitrate etc. on the fly. So set the bitrate to the final value and make use it's using 44.1 khz aswell since this also might mess up the export.