PAM8009 help understanding schematic
The pot between Vdd and GND to pin 7 is to provide a DC voltage to control the volume, with the gain responding in 64 steps, as detailed in the table in the datasheet. I don't see an input resistance or bias current spec in the datasheet, but the use of a 50K pot implies that it's pretty high impedance, so a 10K to 50K (linear) digital pot could be used. You might also be able to use PWM from a microcontroller provided you filter it to low enough ripple and maybe buffer it. Obviously, a voltage-output DAC would work too.  

The 100K is a pullup for the input. The series resistor is probably for isolation to control noise or transients. It's shown in the datasheet for the PAM8019: 



Again, it's not clear how much leakage can be present, nor did I see it explicitly stated that there is no pullup on the chip, but probably 10K - 50K is a safe range of values. You can always measure a sample to confirm no pullup. 

I'm not convinced this datasheet (or at least the info on it)  originally was written in the English language, so there may be other things missing or confusing.