I found the reason of the microphone problem. If you have problems with BV5000's microphone sound quality try this:
1. Disassemble the phone
2. Take off the frame from the display (phone takes off in two halfs)
3. Take off 2 screws on the bottom black trim, where's the mic and the vibration motor
4. Peel off cautiously the small PCB, and THE PROBLEM IS ONLY BAD ASSEMBLY QUALITY:
The small rubber cover on the mic is not in good position, half way slipped, the very small hole on the mic covered them.
5. Take the small rubber mic cover in good position, and assemble the phone.
FORGET THE MTK ENGINEERING MODE, AND MIC SENSITIVITY LEVELS!