Internal mic volume too low in Android Head Unit (Fix)
Here are microphone level values
Level 0 - 220 Push Set (64)
Level 1 - 220 Push Set (112)
Level 2 - 220 Push Set (192)
Level 3 - 220 Push Set (112/144)
Level 4 - 220 Push Set (192)
Level 5 - 220 Push Set (192)
Level 6 - 220 Push Set (184)
Level 7 - 220 Push Set (184)
Level 8 - 220 Push Set (184)
Level 9 - 220 Push Set (184)
Level 10 -220 Push Set (184)
Level 11 - 0 Push Set (0)
Level 12 - 0 Push Set (0)
Level 13 - 0 Push Set (0)
Level 14 - 0 Push Set (0)
the above trick might not be 100% fix but if RCA cable is replaced and an external mic is added then it will give 100% improvement in mic volume.