Start Sound / Audio in Backtrack

--> Open the terminal
--> Type sudo nano /etc/group
--> Look for the line audio:x:29:
--> Add your username after : eg. audio:x:29:hackernekar
--> Reboot

An alternative approach is to directly browse to the file /etc/group, right click it and select edit as root. Look for the line audio:x:29: and add your username after : eg. audio:x:29:hackernekar. Save the file, exit and reboot.

Now you can enjoy sound on your Backtrack OS.