Sound Chip Audio Amplification Circuit YM2612 + TDA1308
The polarity of the audio coupling capacitor C4 is backwards which causes a DC unbalance and the amplifier amplifies it.
Two of the many coupling capacitors in the Mega Amp mod circuit are also backwards.

The audio gain is 240k/6.2k= 39 times which is crazy since the audio input to C4 is probably at least almost 2.8V peak to peak.