I found this interesting article over at droidhacks.org by droidbot that i thought might be useful for some people.
"Depending on the sound quality of your Droid you may or may not notice a difference. Some people have reported to have better quality on the default setting then from the custom setting. Some have also reported that the custom setting makes them sound crystal clear. Use at your own discretion.
Step 1 – Dial ##7764726 (##PROGRAM) and press SEND
Step 2 – Enter 000000 (six zeros), for the SPC Password and press Verify
Step 3 – Press “04 Vocode”
Step 4 – Press EVRC-B then Apply
Step 5 – The phone should automatically reset a few seconds afterward. If not, power cycle the phone.
This changes how the phone compresses and decompresses the call audio which improves the quality"
edit:To undo just follow steps 1-3 again and instead of pressing EVRC-B select EVRC then apply