I can’t help but wonder if this is a hardware issue, not a software one.

First off, if you are using the jack rather than HDMI, I think I know where you are getting the humming sound from. Are you able to use the HDMI output for sound, or is this a cabinet (or old TV) installation? With my home-made cabinet, I am using a new style tv at 90^ because the audio jack and crt/projector video seems to throw too much interference for the sound to be crystal clear. But the hum does make it sound more authentic…

I have a set of powered speakers that I was going to use on my cabinet that produced almost identical issues. It picked up a hum from the projector within the cabinet (foil fixed that issue until I figured out how to turn the video output 90^ and grabbed a new flat) and it was crackling on specific heavy load conditions where the driver (coil of the speaker) was under heavy stress.

After a a while, I added an 12v light strand into the power circuit for my speakers and draped the inside a reflective cabinet in the top behind the PAC MAN translucent so that it would light up when the machine was on. This allowed me to SEE the amount of power consumption on the power circuit. When the cracking started, the light was either out or dull and flashing like a weak strobe light. Turns out, there just was not enough juice (with the power supply that came with the speakers) to handle the load, and no it was not because of the lights. I removed the lights and took out my trusty multimeter to confirm. I switched them out for a sound bar that I modified (as I had installed a flat screen).

I have no hum and no crackling. Though now I miss the humming because it made me feel 14 years old again with the humming of the arcade cabinet… nostalgia…

I would try a different set of speakers to see if the issue persists. If the issue is worse or better, I think you can bet the speakers you have are underpowered (as far as cracking). As far as the humming (and assuming you are using the audio jack), I would try moving the speakers (and wires) around to see if there is any change in the intensity of the humming. interference can be picked up by the coils, but also by the wires of the speakers.

OH, and could you please also let us know what your set up is currently? RPI version, monitor type, speaker set up or rather what you are using for your audio outputs… In this situation, it COULD be just a hardware issue. Or not. :)