To calculate an approximate value for the filter capacitor, it helps to make some assumptions first. Lets assume that your power supply rectifier is a full wave type. This means that your ripple is dominated by the 120 Hz component. Let's also assume that your load current is constant and not jumping up and down rapidly. This assumption may not be spot on for an audio amp, but if you start with a load current that represents the maximum output power of your amp, at lower output powers your ripple will be better since the current draw will be lower.
Now, to estimate your capacitor value, use the equation
CxE=Ixt
where C is the capacitor in microfarads (not farads)
E is the voltage droop from the peak of one rectifier one cycle to the next. We approximate by calling this the peak to peak ripple voltage.
I is the load current in amps
t is the time between the peaks coming out of the rectifier, in milliseconds.
For example, let's say you have an existing capacitor you want to use and you want to know what ripple to expect. Let's say your output voltage is 13.8VDC and your current draw is 2 amps and you have a filter cap of 5000 uF. Use the formula to find that E, the ripple, will be 3Vpp (since t is 7.5 mSec in this case). This is a lot of ripple, so a larger cap is needed. However, if you included a linear voltage regulator after the cap that had an output of 10 volts, then you would get a very smooth 10V.
This is just a way of estimating where to start. After this, some experimentation is the easiest way to fine tune things.
Of course, in order to estimate your capacitor, you have to guess at how much voltage ripple is acceptable, and this will depend on the "power supply rejection ratio " of your audio amp.
By the way, this theory and example are taken from the ARRL Handbook 2001 edition, page 11.11.