I don't know what mike you are using, but Icom, especially the older ones, need a high output to function correctly.
Most are electret amplified and have + voltage on the mike lead to provide the operating voltage.
Yaesu and Kenwood have a different arrangement where the + voltage is not on the mike line and the power is coupled to the mike amplifier in another way.
You might want to try a full reset to see if that helps.
Have you tried both mike jacks? Rear and face panel?
