keywords and mp3's

Do you mean anchor text that is linked to your mp3's?

You should make your hyperlink have anchor text that is related to the content of the interview.

e.g. <a href="http://www.websitename.com/interview1.mp3">Interview with list building expert John Doe</a>

The text "Interview with list building expert John Doe" is your chance for keywords. What you can do is use that same link with different anchor text in different places. If it's on a page that has little text then stick to only one hyperlink, if it's a medium size page then 2 hyperlinks is probably ok, if it's a particularly long page then even 3 hyperlinks is probably ok.

Just remember to make the hyperlinks appear as part of the flow of the page. If you've just made a particular point about "green boxes", you could add a follow up line to say and to learn more about green boxes listen to this interview with <a href="http://www.websitename.com/interview1.mp3">Green Box Expert Maggie Smith</a>.

You could also have links to the interview from other pages as well, just remember to keep them in context. If you've maintained a "silo structure" to your site then make sure you only include links to the interview from pages in the same silo.

Have like an interview page with all the interviews from your site on that one page and make sure this page is referenced in your site map. There are plenty of options, just remember to vary the anchor text, keep it in context and natural and don't overdo it.

I dont know another method than Anchor text unless if you are embeb the mp3 and add alt text.

I would edit the files metadata to include your keywords. Just right click on the file and go to properties, then find the Details tab and fill out the fields. Then re-upload those files so you replace the old ones.