The code you'll want is:
HTML Code:
<a href="http://www.filefreak.com/pfiles/8602/SAMPLE.mp3">Click here for sample clips of my work</a>
[you may edit the text if you wish].
When adding or editing your eBay auction, when you get to the part of your document where you want to add the link, simply click the "HTML" tab at the top of the text box and paste in the code above.
Then, click the Standard tab to switch it back to "Normal" view to continue editing.
Let us know how it goes, and if you need anything else.