Allowing users to create audio directly on the site? (Similar to BlogTalkRadio)
Using Jamroom
I believe you would have to run an RTMP flash server to handle the incoming audio and "record" it - I think Red5 is one:
http://www.red5.org/
I've never used it, so can't comment on how it works, but that hopefully would get you started in the right direction.
Hope this helps!
