Measuring RMS power output from audio amplifier channel
I think you are going about this the wrong way. It's not clear whether or not you have control of the signal source. The approach might be different for a fixed CD player and quite different if the users can use their phones, for example.
My approach would be to choose a piece of music with a reasonable volume level, set the output on the device to max and then adjust the amplifier gain to a suitable level and maybe a little more to accommodate quiet pieces. While adjusting listen to the output for signs of distortion.
Of course, before you start you would check that you have matched the speaker and amplifier impedances before you start.