The two black discs next to the big capacitor with the phillips/flat head. Top right quadrant of the board, as it is installed in my machine (lower right quadrant in picture).
The left one in the picture controls the squawk volume and the right one controls the talk volume (clockwise is louder and there should be a stop at top and bottom). Do the same for the pot on the coin door. Sometimes just flipping it back and forth will at least let you know that the board works. If you are hearing a hum or static, then your speaker is not totally dead.
If you start getting sound when you operate the pots, then use air (or contact cleaner from what some people say) to clean the pots and see if you are back in business. If it is still intermittent, consider changing the pots. I had no talk on my board until I went low/high volume about 6-8 times and used clean compressed air (canned air, if you will) to blow out the pot. I knew I was in business when the talk was initially intermittent and faint but continued to clear up as I "cleaned" it.
Also, wiggle around the connectors a little bit to see if you can find a pin bank that has a solder joint that is going bad. Sort of a DIY television repair method, but it can help narrow down things in a hurry if you jiggle something and it starts to work at least intermittently.