I understand what you're trying to accomplish, but I know of no way to do this. The basic reason is that an mp3 is essentially identified by applications in it's entirety. For flash to index into it, you'd have to embed certain tags into the file and reencode it as a swf movie. Far more time consuming than chopping a piece of the file off.
There might even be php libraries available to do that sort of thing similar to the image libraries. I am not familiar with any yet though. Keep us posted on your progress, this topic interests me.