I found a SOLUTION for the crackling on 16" 2021 M1 Macbook pros which might be the same case for many MANY people with the same model
It was not software related NOR hardware 'failure' but something very simple. This is linked to a slight flaw in the design, which is not really talked about.
PROBLEM: Crackling and popping on Right OR Left speaker during "bassy" sounds. Software updates and sample rate changes fix nothing.
Note: Generally, software issues will happen on both sides as the sound engine should be sending the same signal to all speakers.
Solution: Clean your the grills on the sides of the 16" M1 macbook pro & if necessary, the subwoofer.
How I came about this issue:
I bought an 2021 M1 16" only a few months ago because I heard of such great reviews and it was second hand (only used once by a company then packed up). I actually LOVE it.
To my dismay, the crackling and popping started and I was distraught after spending the money. Software updates and changing sample rate did absolutely nothing.
Thankfully I still had about a month of Apple Care left and so I sent it over to apple thinking the speakers had popped or something.
A week later, I get my M1 16" back with a note from the Apple Repair Centre stating that "Dirt was cleaned" as the "Fix" to my problem. That was surprising as it's not in a particularly dusty area.
But it worked and things played perfectly for a well with the audio... at least for a few months.
Then the crackling started again on my lower right speaker.
I figured it was the BASS speaker which is located on front right area of the 2021 M1 16" right where there is a grill.
Apple Care had ran out and I wasn't going to send this all the way back and pay for it to be "cleaned". So I decided to check how to (safely) remove the bottom case to check the bass speaker and got all the tools.
Low and behold, the right bass speaker had a couple tiny stones/grains of sand or debri. I brushed it gently off and used an air spray to dislodge any debri then closed the case.
The Crackling and Popping disappeared once more!
I do hope that in the newer models / future models that Apple address this flaw as this was overlooked in the design for sure.
The grill next to the bass speaker is wide open for any debri to drop in it. Once lodged in the subwoofer, you get a wonderful crackling from the vibration of the diaphragm. There are no filters or anything to stop small bits of dust of debri from slipping in - say you are working outside and wind blows something in or if your travel bag stores the laptop on it's side and debri falls in the unprotected grill.
Most laptops would some form of spongy filter - perhaps this was never thought of or ommitted to improve sound dispersal. Either way - it was a bad idea.
I hope this helps someone! If you have apple warranty, please send it in and don't open it. It's not worth voiding warranty unless it's out of it AND Apple will realise people are having this problem. It's an expensive piece of kit to have such a simple but annoying flaw.