Distortion gone. Clear channel sound restored!
I had my MacBook Pro 15" Retina (Late 2016-2017) upper case replaced back in 2019 by Apple under service recall for BOTH bad butterfly keyboard design and problematic battery. Alas, 3 years later the battery went bad again (despite an expected lifespan of 5-6 years). I did the very difficult battery pack replacement following the iFixit guide. Success.
Alas, only a month after battery pack replacement, some keys started going wonky again: repeating characters, totally annoying. Despite valiant efforts to clean the keyboard of debris, I ended up with a totally broken I key (as seen in photo). So I ordered a "B stock" upper case from iFixit and followed the guide. I was able to transfer the new battery from the old upper case to the new one using iFixit adhesive remover (acetone). Put it all back together. Keyboard 100% functional again.
Alas, I somehow damaged both speakers in the process. They buzzed and had only high frequency response.
I ordered the left and right replacement speakers from iFixit (being all too wary of what I might get if I were to oredr from Amazon or eBay). This final step in my journey was quite easy. But do note in the photos that the adhesive on one of the speakers started to pull off unevenly. I was loft with a gob of curled up adhesive in just one small area. Nevertheless, I pressed the speakers firmly into place and buttoned it up. Speakers sound great! Back to full frequency response and no buzzing. I'm happy. Are you happy?
Don't do just a battery replacement. Buy the entire upper case assembly. Beware of any MacBook Pros (2015 through 2019) that have this dreaded butterfly keyboard. Worst keyboard Apple ever made. Supposedly this has been rectified in the latest MacBook Pro models. They went back to the more reliable scissor switch keyboard mechanism. Moral of the story: thinner is not better. I'll take my MackBook Pro thick and well done.