Saw this tip over at HoFo:
Get into the programming menu, from the call screen, enter ##PROGRAM and press send. Enter code 000000.
In option 04, you can change the vocode option to EVRC-B, it made my in-call audio sound better. Your phone will reset after you exit and apply the new setting.
Warning: Be careful what options you change in the programming menu, you could potentially brick your phone.
Also, be aware that there is a noise cancelling microphone on the back of your Droid. Look for a small rectangular slit/hole near your headphone jack on the middle, back edge of the case. If you cover this mic with a case or your hand/finger when talking, it could cause audio problems.