Where would be the best spot to put a logarithmic volume control on this TPA3122 audio amp?
Hmmm the precision of an analogue pot eh? Compared to the digital nature of a 15 watt digital class D amplifier. So be it.

Lowest input impedance of the chip will be at max gain and this could be as low as 9kohm so use a 1, 2 or possibly 5k pot on the input - wiper to chip and pot low end (CCW) to 0V. Input from external input to pot CW pin. Stereo pot of course.

Here's the data sheet for reference.