@metalov: Your are correct. I tried it and the mp3 files do not appear in the Media Player. However, the creator of tracker-cfg/ was kind enough to send me the following by e-mail:
"
Add all the directories you don't want indexed to NoWatchDirectory, save and rebuild. Wait.
If you want to exclude specific files add them to NoIndexFileTypes.
"
I did not try the NoIndexFileTypes yet as you cannot browse files, but will try it out when I have some more time.