Can you be more specific about "there was audio playing" ?? What was connected to the input?? Many audio systems can detect AM radio broadcast signals and produce an output. That is quite common. Sometimes it is a serious problem.

The MAX98357A was connected to this bluetooth receiver via an I2S interface...

https://www.aliexpress.com/i/1005005428336902.html

I connected the bluetooth receiver to my phone and played a 1kHz sinewave

 

The circuit in post #7 can still be the simple effective choice. It may require different resistor values for the best performance, though.
BUT, with the specific application, and using only one speaker, I see the possible problem.
THEN I looked at the photos of the speakers in post #7! each speaker cable ends with a STEREO 3-circuit plug. My guess is that the two speakers only connect to the sleeve and eather the tip or the ring. So the situation is a bit different because there is no two-conductor plug involved.
Am I the only one who has noticed that??

 

The circuit in post #7 can still be the simple effective choice. It may require different resistor values for the best performance, though.
BUT, with the specific application, and using only one speaker, I see the possible problem.
THEN I looked at the photos of the speakers in post #7! each speaker cable ends with a STEREO 3-circuit plug. My guess is that the two speakers only connect to the sleeve and eather the tip or the ring. So the situation is a bit different because there is no two-conductor plug involved.
Am I the only one who has noticed that??

Yes there are 4 possible combinations, the diagrams below show a cross section of the stereo socket so you can see where the connections are made....

1. Mono jack with single speaker:




2. Stereo jack with both left and right speakers:




3. Stereo jack with left speaker only (right speaker has been cut off):



4. Stereo jack with right speaker only (left speaker has been cut off):




My current configuration is number 3 (stereo jack with right speaker cut off).

Originally the amp negative was connected to the socket sleeve and the amp positive was connected to the socket tip, but this meant only configurations 1, 2 (left speaker only) or 3 would work. Configuration 4 will open circuit the amp.

Now the amp negative is connected to the socket sleeve and the amp positive is connected to the socket tip AND ring, but this means only configurations 2 (both speakers), 3 and 4 will work. Configuration 1 will short the amp output.

 

I still can't make sense of the PWM waveform, I wanted to understand it so that I can calculate the current.

I found a change over switch 'CL-SB-12A-02T' that I could try to squeeze onto the PCB and its rated to 200mA...

https://www.nidec-components.com/eu/product/detail/00000191/

If the voltage across the speaker is only 5V, then this means a speaker resistance of 25 ohms or more would work with this switch ( R = V/I = 5/0.2)

The amp datasheet specifies a power of "3.2W into 4Ω at 5V" which works out to be 640mA ( I = P/V = 3.2 / 5).

But the amp datasheet also specifies an absolute "Continuous Current In/Out of VDD/GND/OUT: ±1.6A".

I'm not sure what current the amp can actually provide and whether that switch rated to 200mA is suitable.