TL;DR: This is a story of how I felt nostalgic for the old Uh-Oh! ICQ sound and ended up with a quick research on how Slack stores and uses media files. Eventually I ended up developing a simple tool to customize Slack sound notifications. Please note that currently my script edits the Hummus notification sound but this can be easily changed if needed.
Interesting. I found that I was able to just replace the sound files without any reverse engineering. Then again, I use the web app.
The first thing I do on every new machine is to turn off all and every kind of notification including sound - especially Slack notifications those drive me insane...