Well, this is another textbook example of "nothing is so easy as..."
Identifying suitable component parts to do this is easy. It's the design of the circuit and assembly that requires the mind's imagination to go into overdrive! For example, if I were doing it I'd probably just use the audio amp taken from a surplus PS2-5V board.
It has DC-volume-control and puts out 5 Watts as I'm imagining I'd want to crank up the 1/2 Watt output from the sound module. The part is now obsolete but then again I'd only be building a one-off. It would take a dozen or so other inexpensive components (resistors, capacitors) but a lot of soldering and experimentation. I figure at one time there was an evaluation module made by the manufacturer with the TDA7056 and auxiliary components that would vastly simplify the job, but that time is history. I found some new audio amp chips with DC-volume-control that have an evaluation module but they were very expensive (over $20) and of course would still require some fiddling to make them compatible with the signals and voltages in the turntable drive.
