is right - although you can simply use your finger nail.
I just fixed this problem and the low volume was annoying me for days!
The trick is to use the media player to play a song and while it is playing, apply slight pressure with your finger nail to the top of the gray rectangle with the camera flash in it.
Hey presto! Volume restored! No need to take the BB apart or risk voiding the warranty.
Thanks to all who posted this fix