THIS IS NOT THE T-MOBILE AUDIO FIX!!
You need to first download a builp.prop editor app. This one works great: https://play.google.com/store/apps/d...ld.prop.editor
Open the app and change the values of these lines:
persist.audio.handset.mic.type=digital --> replace digital with analog
persist.audio.dualmic.config=endfire -- replace endfire with false
That's it! Reboot and enjoy! This does not fix the speakerphone echo issue, but I'm pretty sure the solution is editing the build.prop in another spot.
Here is the original thread