This is common to Android builds on HTC HD2, you can find many posts that refer to this issue as "Robot voice" or "Robotic voice". The following fixed the issue for my phone, but I don't know if it will work for all builds.
You need to have SuperUser permissions to perform the following
- Rename or delete '/system/etc/firmware/default.acdb' file
Also check the following threads on XDA.
Background noise fix / mic gain fix
Background Noise and Echo - fix