I just woke up to the fact that I'm hearing the same sound that kevs is complaining about, also with user interface sound effects turned off: it's system sound "Basso."
I hear it if I inadvertently hit command-H when focus is on my desktop, and I also hear it in Music and elsewhere.
It's a non-specific alert, and I"ll go along with kevs that its being audible is a bug...an annoying one.
