Just heat the joint for around 3 to 4 seconds until the solder melts and reflows heat the solder pad and component leg at the same time to get a good electrical connection, just for info the audio path is highlighted here in blue
Where is it you actually lose audio? The only part of the circuit you should get audio is in the aptly named ‘audio’ section in purple below
Here are the voltages you should be looking for.
Power Supply 9V
G +0.5V to -4V
C 0V – 7V