How to calculate the required battery capacity for audio system
Your real question is apparently how much power does your amplifier and speaker combination draw at normal listening levels, when it is known to draw 200 W peak.

The best answer is simply to measure a few representative cases.  It is really hard to guess how much below the 200 W peak the average consumption is.

This will depend on the volume of the output sound, which is a function of the volume of the input signal and the volume control setting.  How sharply volume effects the power consumption has to do with the architecture of the amplifier.  If it is a efficient class D, then power consumption will be some minimum bias plus a large component proportional to volume.  If it's a class A, then the bias level will be much higher with a smaller remainder that goes up with volume.

Again, you have to measure it.