I answered you on MSBT's comments, but I'll post it here as well in case anyone else happens upon the post.
Are you putting the extension (.wav) on it? The sound file must be a .wav or .mp3 file. See the FAQ linked via MSBT's description page (
http://mikord.wowinterface.com/portal.php?id=156&a=faq) for an entry ("How do I add my own custom sounds?") for step-by-step instructions on how to add custom sounds.
Also, as Seerah pointed out MSBT works seamlessly with SharedMedia, so anything registered there will be consumed. Similarly, any media added to MSBT through its in-game media editor is exported to SharedMedia for other AddOns to make use of as well.