I recently had a problem, and resolved it, while using Shotcut, and I wanted to share because, even though this topic is a bit older, it seems @finleyfooonfire and I were having a similar problem!
The problem I was having was that Shotcut would not play ANY audio, no matter what format I converted the file to and tried again and again. I searched the forums and found this topic: https://forum.shotcut.org/t/no-audio-within-shotcut-at-all/9712/16
I went through all the suggestions given to the OP of that topic, and still had the problem. At my boyfriend’s suggestion, I looked at the system audio, and on my Linux system, in the lower right, where there’s a speaker icon, I clicked on that, and volume was muted for Shotcut for some reason. My boyfriend’s thinking, and I agree, is that I probably accidentally tapped a hotkey.
So, hopefully the shotcut user in the above topic resolved the issue somehow, but I thought I would offer the solution I found. It wasn’t shotcut at all!
One main difference I found was that when I connected a speaker to my computer with a 5mm jack, audio still would not play (because it was muted by the system) whereas finleyfooonfire was able to play audio that way. So it sounds like it may well have been a different issue.
Hope this can help someone!
edit to add: the audio worked in other programs (VLC) and was only muted by the system for shotcut.