Purpose of capacitor in audio amplifier circuit
Given that the corner frequency for the R46-C46 combo (\$\frac{1}{2\pi R C}\$) is about 50 kHz, it's probably there just to make sure that the circuit doesn't oscillate at ultrasonic frequencies.



EDIT (responding to questions in comments):


  You said, “a capacitor from Q12's base to ground would have an ill-defined effect, because the parallel resistance (the effective collector resistance of Q10) would be varying with the signal voltage.” Does the diode, D5, prevent C46 from having a similar ill-defined effect on the effective collector resistance of Q10?


No, and perhaps I shouldn't have said that. The reality is that the effective collector resistance of Q10 is very high relative to the other resistances in the circuit (it's a current source), so its effects are negligible. I should have just left it that a second capacitor would be merely redundant.

The impedance of R46, R49 and D5 are all much less than the collector impedance of Q10.


  If so, is that the primary reason for the inclusion of D5?


No.


  If not: 1) Does C46 cause a variation in the effective collector resistance of Q10?


No, the collector current of Q10 is simply a function of the signal voltage.


  And, 2) What then is the purpose of D5?


D5 is there to provide some of the bias voltage for the two sets of output transistors. The bases of Q11 and Q12 need to have a DC offset between them in order to eliminate crossover distortion in the output stage. This DC voltage is developed partly across R49 and partly across D5. The advantage of having D5 in the circuit is that it has a temperature coefficient that is similar to that of the transistors, (partially) compensating the bias for temperature variations.