That dent won't affect the sound at all, just a dust cover. There are a couple of methods you can try..I've had good luck using a vacuum cleaner attachment on a lower suction setting. Just move slowly towards the dent and often it will pop right back. Or, Axiom has a video using masking tape. I've even used a small strip of duct tape to do the same..

http://www.youtube.com/watch?v=xFbvhokVrPI

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