You would just need it to be hosted somewhere online (and a player script, I guess) and then point to it in your listing's HTML. I've never done it myself though.
Personally, automatic audio that I didn't ask for in listings annoys me and I'll either back out of it or see if I can quickly shut it off.
Of course, if it has to be played manually by the viewer that's not so bad -- except that it will slow down the load time of the page.
Also, I'm not completely certain if it's actually allowed at eBay.