Capacitor C1 regulates the low frequencies (bass), as the capacitance grows, the low frequencies are getting louder. Capacitor C2 regulates the higher frequencies (treble), as the capacitance grows, the higher frequencies are getting quieter. This is a class B amplifier, this means, that a current must flow through the end transistors, even if there is no signal on the input. This current can be regulated with the 500ohm; trimmer resistor. As this current increases, the sound of the amplifier gets better, but the end transistors are more heating. But if this current decreases, the transistors are not heating so much, but the sound gets worse.