If you can barely hear the person you have called, try these 2 things.
Go to settings > call settings >> unclick the Voice Privacy check.
If that doesn't work, remove and reinstall the battery.
This has worked for me. this issue, alone, makes me really hate the droid, and seriously causes me to rethink my desicion to purchase this phone rather than the i-phone.
I just had to get a replacement droid, because my volume was completely gone, and I had to use the speaker on the phone to hear my calls.