I have been looking all over for how to customize the notification sound* and tried replacing .mp3 files in a few places. The following is for MacOS—I've tried changing in the Applications folder, /Applications/Slack.app/Contents/Resources. Specifically, I got my desired .mp3, renamed to an existing file name, added to the folder, and replaced. This did nothing, after clearing Slack's cache and restarting Slack and comp.
Then I tried looking elsewhere—I found /Users/[username]/Music/Music/Media.localized/Unknown Artist as a folder that contained all the Slack file names. I wasn't able to replace the files, so I instead edited the .mp3 in quicktime and saved that edit. To no avail.
Anyone have any recommendations or resources to link to?
*I really just want to hear Navi say, "Hey, Listen!" every time I have an urgent message. Since Slack was originally a video game and its sounds are seemingly from that game, it seems appropriate, right?