Disable Phone's Built-in Microphone?
Maybe disabling the noise cancellation feature might work..

Get a rooted file editor first, then
from "/system/build.prop" change the 
"media.a1026.enableA1026=1" to "media.a1026.enableA1026=0" 

Then do a reboot. See if your problem is solved. I simply suspect the android's mic always gets enabled for the purpose of noise cancellation during calls/voice input.

REMEMBER! I do not hold responsiblilty for any failure. I suggest you make a backup and everything when you try this.

SOURCE: http://forum.xda-developers.com/showthread.php?p=6787214