Microphone preamplifier
No, the microphone pre-amplifier circuit is not biased class C.

The transistor is certainly biased on and, assuming the microphone signal is small enough, the transistor will always be conducting so this is a class A voltage amplifier.

Why do think it is a class C amplifier?

There isn't much to explain about the operation.

A bias voltage is applied via the resistors \$R_2\$ and \$R_3\$.  The operating point is stabilized by degenerative feedback, i.e., if the collector current increases, the collector voltage decreases which decreases the forward bias decreasing the collector current.

This type of bias is described at Wikipedia article section "Collector feedback bias"