New kind of glitchy audio playback issues

Hi guys, I am getting pretty desperate here so hoping someone can help!

I used to have to deal with glitchy playback when I didn't have enough RAM or my hard drives were too slow, but this is different. 

When playing the timeline my audio is glitching like crazy. It's like dealing with a scratched record with random skips and glitches. Eventually the audio will appear to be out of sync unless I stop and start again. This happens with all forms of audio, including my natively recorded narration. I am not certain but I am pretty sure it doesn't happen with the audio in the video files themselves. There's nothing wrong with the audio and there's no consistency that I can see with when it happens. Sometimes it won't happen for ages. 

Changing to Better Performance doesn't make any difference. The image is fine, no glitching there. Just the audio going screwy. This happens if I play full screen, or just playing the timeline. If I export, everything is fine when played in QuickTime. 
  

SPECS: 

My files are mostly WAVs for audio, with the occasional MP3, while the video is either MP4 or MOV.  

Content is held on external hard drives (4 x 16tb Lacie Big Raids daisy chained)

Big Sur Version 11.1
Specs: iMac (Retina 5k, 27-inch, 2020)
Processor: 3.8 GHz 8-Core Intel Core i7
Memory: 64GB 2133 MHz DDR4
Graphics: AMD Radeon Pro 5500 XT 8 GB
FCPX - 10.5.4

Sounds strange that the video is playing fine. Have you tried trashing your preferences?

Not sure if this will help narrow it down, but what happens when you create a new library and import just one audio file that isn't playing properly and put that on a new timeline by itself?

Re-encode those files in Compressor.

Thanks for replying guys, I appreciate the effort.

Here's a video with an example of the glitching. It happens on every file, across every project, across every library, whether that's on my desktop or external hard drives! You can hear it on the words PUSHOVER and RECENT YEARS. I can't re-encode every audio file, that's a non-starter. Not to mention that lots of my audio is recorded natively in FCPX! 

What video?

What is the frame rate, resolution and video codec of the files you are editing in your timeline? AND....what is this same information for your exported files that play correctly in Quicktime?

Hi thanks for your previous replies - I ended up living with this issue for a while and more recently came to the conclusion that it's the audio presets that are causing the issue. The files play fine as long as I don't drop any of me preset mixes on. I would love it if someone could help me figure out why that is!