So basically you can't use the phone to talk then...>_>;
1)Did it come like this when you got the phone
- might be a defective phone
- you can try a factory reset
2)was this something recent?
- might be an app you downloaded that's affecting the mic.
- do you have one that you recently downloaded that would have access to the
mic?
- if you don't know...you can slowly do the 'uninstall and reinstall' and test to see
if the mic work after each uninstall.
- if you can't pinpoint an app, do a factory reset
- and if issue still persist, call HTC
Hope you get this resolved

-Roze-
Have you done a fac