LM393 comparator misbehaving and causing issues
The output of the comparator will swing many volts with a distorted version of the audio input signal. It will have very fast rise times that can easily couple though stray capacitance to other parts of the circuit.
I would suggest putting a capacitor of a few microfarads from the output of the comparator to ground. A small resistor (eg 47 ohm) in series with the comparator output will limit the current when discharging the capacitor.
That will do two things;

When triggered the comparator will discharge the capacitor via its open-collector output and stretch the signal so that it will light the led brighter than the very small duty cycle that you currently have.

The output signal will now change slowly and not couple into the audio output signal.


I use an almost identical circuit to turn on my power amplifier in a house audio distribution system for the last ten years.