thats its not forum byt RADIO
You need MANY MBs of hard disk to save the files.i thing its possible byt its better user upload the own file and with simple BBCODE to add it one one player on to the post!
(User record the message on his computer and then upload it)
After user post it in to your forum with player bbcode like [player]htt://mp3.mp3/mp3file.mp3[/player]
EASY WAY...