What you need is :
- Add a wire in your Power Pak between pin 51 (EXP9) and 54 (EXP6). The reason pin 51 (EXP9) was chosen was because this is the only pin which is unused in the Power Pak, and that pin exists in the toploader (pin 54 simply doesn't exist).
- Add a 1.2 kOhm resistor between pin 51 (EXP9) and audio out. The audio out pin is right next to pin 51 so it's really easy to solder the resistor here.
- If you want audio with actual FC games, you'd have to add a wire in the 60 -> 72 pin converter, between pin 46 (60-pins FC side) and pin 51 (72-pins NES side).
On a side note, I noticed the audio in the toploader is not amplified at all, it just comes straight out from the 2A03, through a set of resistors and capacitors. That's why I have to make the volume louder to compensate for that (as opposed to, for example, the SNES or toploader NES).