c902 very low mic volume

Just bought a new c902 from a guy, but i now realize that in a phone call my microphone isn`t working ok. If i turn on speaker or head set the microphone works ok. I have even reflashed it using seus but still my microphone isn`t working. also in test mode microphone is ok. What can I do now?
I have changed all acoustic files in my phone and if speak realy loud the other person can hear me realy low. So there is no hardware fault I guess. Probably the guy I bought the phone from flashed the phone before selling to unlock sim.
I have reflashed my c902 with an old firmware today and also with the latest firmware. The problem remains: in normal phone call i have to speak really really loud for the other person to hear me. When i turn on speaker everything is ok. I have disabled the up key, and even tried to keep C butoon pushed but still no change. Is there any way to increase my microphone volume with like 300-400%?