I am going to assume that you mean you are playing music off your disc player instead of mp3s, as this is a common problem for people with laptops and desktops.
The optical drive and belt get extremely hot when run for long periods of time. If you've been doing it for about 3-4 hours straight, you might want to reconsider what you are doing.
If it is actually less time than this, or if it just smells like this when you play mp3s or other hard disc files, go and submit it under the warranty