I'm having the same problem with the loadSound(). No matter what I try the mp3 download continues and clogs bandwidth. Here's what I've tried with no success.

1. loading a new mp3 into the sound object. Deep is right the position goes cumulative.

2. Creating a new sound object over the old.

3. Creating the sound object inside an MC and unloading the MC

4. Creating the sound object in a level and unloading a level.

Even if you leave the url with the flash embeded the mp3 file continues to download.

The only way I have found to halt the download is to close the browser or projection file.