yeah, as far as drilling holes in the battery cover goes for me... i wouldnt try it. but what i did do to fix this problem a long time ago was take one of my business cards, cut the corner off (to just over the size of the speaker) and used a very small amount of super-glue on the corners of the cut card and placed it over the speaker. it solved the problem and doesnt muffle it any. if it does, i cant tell and neigther could my wife.

its a safer alternative and if you ever need to warranty ur phone, you can get the new battery cover too. not to mention that any dirt/dust/lint wont get in thru the holes.