swivel cuts the audio of mp4 flash animatio after a few seconds or minutes (i think)

i noticed this error before but when i encode an swf longer than a few minutes (i think so?) into mp4 using swivel, the audio cuts off after a while in the encoded mp4 file

this never rlly bothered me but i feel like wanting to fix it now, so is there any way to fix this error?

cheers