I had the same issue with my iphone 4. One day the volume seemed low and muffled. The sound was fine with an external headset or blue tooth so I knew it had to be in the speaker. In my case I think pocket lint and dirt ended up clogging the ear piece. I used my air compressor and about 90 psi of pressure and blew out the ear piece, speakers on the bottom, and the headphone jack. I was surprised at how much dust came out. Since then the phone has been like new. Just make sure that your compressor doesn't have any condensation in it. In my case I actually blew a little moisture out from the speaker area. It was hot here one day and I was using the phone. I believe my sweat and lint may have been the issue. Hope this helps someone else.