I don’t know how I stuck with the super annoying default notification sound for a successfully expanded Text Expander snippet for all these years, but I finally got around to finding a suitable and non-intrusive replacement.
Here’s the sound in .mp3 format, for previewing it; and the .aif sound file to download and drop into your ~/Library/Sounds/
folder (both are Dropbox links).
Notifications sounds on macOS have to be in .aif format, and it’s a bit convoluted to do it with iTunes.