No, it is not good practice for audio, in fact it causes the circuit to not work.
Imagine there are positive peaks on audio. The capacitor would always charge on the positive peak but never discharge on 0V or negative peaks. The capacitor would just slowly accumulate charge from the audio to have a voltage over it, until voltage is enough that it will never ...
If your MCU will be analyzing AC waveforms, then you can simplify the circuit. The resistance of R1 and R3 need to be large enough so they don't affect the amp driving the speaker, 44k should be more than enough.
You could also eliminate R7 if your cap voltage rating is high enough.
Realize that the two stages are not independent, from an AC perspective, the ...
[...]as long as the voltage supplied to the speaker measures up to
~25Vpeak from an oscilloscope, the speaker can be expected to output
Yes, you should expect sound out. With 25V peak sine wave applied, you could damage the speaker:
from overheating its voice coil wires
from distorting the voice coil form
As Neil says, 40W into a voice coil MIGHT ...
You can use a 70V audio line transformers. One connected in reverse to the amp and then each speaker has its own transformer, everything will remain matched and no power lost is a resistor. You need to size the one at the amp big enough to handle the power of all of the speakers then smaller ones can be used on each speaker.
What you are describing is distortion. Your speaker isn't "breaking." If it broke, it wouldn't work anymore afterwards.
There are a couple of possible causes for distortion in your case:
The battery can't supply the needed current.
Your audio signal is too high.
First the battery:
You are using an 18650 cell to power your amplifier.
You are ...
I'll assume both input ports are connected in parallel (at least that's what makes sense to me), so that you can input power in any of those indifferently - If they have a warning saying you should not connect a power supply in both at the same time, then that's definitely the case.
About the charging and powering up at the same time, you should be looking ...