have you experienced this since day one or did it start recently?
add or remove any programs that affect audio?
have you tried a soft reset?
hold down power button until you see the "keep holding the power button down for 10 seconds..." popup. phone will shut off and keep holding the power button until the phone vibrates and the screen activates. then release. nothing is deleted but this clears out any cache and untangles anything that may be tangled...
give that a try...