lets start with the basics i know some will sound simple but I don't know what you have tried already.
1. While on call use the volume rocker to turn up in call volume.
2. if you have case on the phone make sure it's not covering the speaker on the top of phone.
3. reboot phone.
4. soft reset (pull battery while phone is on leave out for 10 to 20 seconds put back and reboot)
5. Hard reset
From the Home screen, press MENU and then tap Settings.
Scroll down the screen and then tap Privacy > Factory data reset.
On the Factory data reset screen, tap Reset phone, and then tap Erase everything
Warning this will reset phone all data apps and setting will be removed this should be last resort if none of the above works.
I have read of a few people having this problem with a brand new phone and it turned out the phone was defective so if none of the above works it is most likely a defect in the phone.
Please let me know how you make out.
