Have you tried changing the battery door to one that has an opening for the speaker? That way the sound does not have to go through plastic, which tends cut the volume some. To see if it might help you, make a few calls with the back off of the phone.
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I have found that if I am listening to MP3s or watching a movie on my 8330 that the volume for calls is muted after I stop listening or watching for quite some time. I used the same voice call cycle for no volume on calls and seems to bring the volume right back up.
