Hmmm... I can see why you can not add a sound clip within a thread. If everybody add a sound clip to a thread, just think how chaotic it will sound when a user open up a thread. You will have all these sounds playing all at once!!
Ha Ha Ha
I think that is the reason why you can not add a sound clip within a thread. It's restricted for all forums out there I believe. I have never gone in a forum and open up a thread and hear a sound clip from a user.
I suppose if everyone apply a sound clip with a play and stop button for that sound clip, than it might not be so bad for all users.