To save confusion we will work on one channel at a time. let's start with the left channel. I think this channel should pass an audio signal now. Connect a speaker to the output of the left channel via a resistor of about 100 ohms (This is to reduce the risk of destroying the speaker.) It may need to dissipate up to about 25 watts under fault conditions. Inject an audio signal from the headphone connector of a battery powered radio, cassette player, MP3 player etc via a non polorised capacitor (About 0.47 uF) into the junction of R11 and R12. (This is where you injected the DC voltage.) Don't forget to connect the headphone common to the 0vP line on the amplifier. You should hear the audio from the speaker but don't expect it to be very loud due to the resistor in series with the speaker and the fact that the voltage gain of this part of the amplifier is only 6 (This is 6 times not 6 dB.). If this works the next step is to inject the signal into earlier stages.
Les.
Les.