op amp biasing... why are voltage sources reversed biased +/-?
The name of such circuit is a diode clamp or diode limiter.
Assuming the dioded are ideal, they do nothing until they get forward biased, and they only get forward biased if input voltage exceeds the range of power supplies, i.e. goes above positive supply, or below negative supply.
It limits the voltages at the op-amp input to be within supply voltages to protect the op-amp input from getting damaged from accidental overvoltages fed in via audio input.