Power supply requirements for an audio system are far from trivial. The supply must be able to provide sustained power based upon the average current draw of the system. It must also supply peak currents based upon the signal levels, gains, supply voltages and speaker impedances involved. The following table gives a starting point for the power requirements for an audio system. To use the table, add together all of the speaker channel power levels to get an approximate system power requirement. Select the power level closest to your result in the first column. Then select the DC operating voltage in the first row. The intersection tells you approximately what your worst case average current draw will be. For example: We have a system that consists of a woofer at 40W, and two tweeters at 15W. This gives a system power of 40+15+15=70W. For a 24V supply we see we would need a 3A power supply.
For most audio systems playing real signals, you will never draw this much power. Why? Because to get this power output you would need to drive full scale sinusoids at every speaker continuously. Music does not contain that amount of energy. So the reality is you will need a supply somewhat lower than what the table shows you, but at least it provides an upper bound on the requirement.