Wireless Technician for one of the big two wireless carriers. 25+ years of wireless experience. Nokia and Nortel GSM, Ericsson UMTS and LTE equipment. Ericsson, Alcatel, Microwave Networks, Lynx, Exalt and Harris microwave technician.
iPhone 3GS Battery
Make sure the screen that protects the speaker is free of ear wax. It can be stopped up with ear wax and limit the output of the earpiece speaker. About 50% of the ones I have come across this was the cause of low earpiece volume. Use a small nylon hobby paint brush dipped in 91% rubbing alcohol to clean it.