wa 'alaykum salaam wa rahmatullah
Yes it has been asked before. If you think your question is common, it's best to use the Search function at the top of the forum to look for similar threads and see if it's already been asked/posted. In the advanced search options you can even specify from what forum you wish to search it from.
But I'll mention it again here to make it easier: You can add mp3's using the Attachment method, in which the files you upload must meet specific criterion in regards to size... and can only be opened by registered members. The way to do this is to Create a New Thread, or do a Full Reply to a thread, then scroll down to "Manage Attachments" (view picture below) then browse the picture file from your computer.
That's it really! I hope the explanation is easy to understand =) If not, do feel free to use the search function to find other answers.
BarakAllaahu feek
wasalam