Putting music in your auction listings is simple. First you'll need to find a music file you'd like to use. The most common file format for music is a midi file, these also tend to download faster so your viewing bidder wont be waiting too long to hear the music when they look at your listing.

Here are a just a couple of the myriad of sites where you can find free midi downloads-

Midi.com

IFNI Free Music Midi Files

Once you've found the midi you want to use right-click on it and click on save target as and save the file to your computer.
Now upload the file to a web host such as Boomspeed or to the web space provided to you by your ISP (this process is exactly the same as it is for uploading an image).

Once your file is on hosted on the web right-click on it and click on properties and copy the URL (or address) of the file, it will start with http://.

Here is the HTML for putting the file into your listing

<BGSOUND SRC="http://yourwebhost/yourname/midiname" LOOP="INFINITE">

The loop part means that the music will play infinitely. If you just want it to play once then take that part out.