What is opt-cmd-] bound to and how can I switch this off?

Question

In my newish Mojave install on an iMac I experience some odd behaviour:

⌥ Option⌘ Command] plays the alert sound even if it is bound to an action in a keyboard shortcut tool like Keyboard Maestro

And it is only ⌥ Option⌘ Command], other key combinations which (to my knowledge) are not bound to anything. E.g ⌥ Option⌘ Command[ doesn't make any sound.

Also this behavior only seems to happen in one (my main) user account (other accounts - whether admin accounts or not) do not have this problem.

I cannot attribute this to any main application or menu bar app. (And I have quite a few of them!) How did I check?: One-by-one I quit all main apps and then all menu bar apps - still ⌥ Option⌘ Command] plays the alert sound.

How can I find out what causes this alert tone? How can I find out which app/tool/daemon binds to ⌥ Option⌘ Command]?

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Notes:

I care about this because I usually have on ⌥ Option⌘ Command] a particular Keyboard Maestro macro running. This still works - but additionally this unexpected alert sound plays on the shortcut. (And the sound happens even if I have Keyboard Maestro completely quit.)

Answer 1

Shortcut Detective from Irradiated Software's labs page should tell you exactly what is picking up this keypress.

ShortcutDetective detects which app receives a keyboard shortcut (hotkey).

Answer 2

I would go into system preferences > keyboard > shortcuts, and browse through all of the keyboard shortcuts and see if any have ⌘⌥]as they’re keybind. you could also open Automator, and press the record button, press ⌘⌥] and then click stop recording and see what it says that you did.

Answer 3

If you just want the 'bonk' sound to stop then go to System Preferences > Sound and move the Alert Volume slider to the left. In this case you might miss out on other alerts mac tries to tell you.

Answer 4

This doesn't apply to your situation with macOS Mojave, but might be helpful for future viewers.

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If you're using macOS Catalina (10.15) or newer and you're wondering about applications that might be intercepting specific key combinations, look at the list of applications that have the Input Monitoring permission granted in Security & Privacy settings.

That, combined with the standard keyboard shortcuts, should account for all the different ways that global hotkey keyboard shortcuts can be installed.