How can I add custom sound effects in OSX 13.5

I repeat (sorry, but) put your custom sounds in your user's Library. ~/Library/Sounds

Not in the System. No need root, no need to disable SIP.

The sounds are available in System Preferences -> Sounds -> Sounds Effects like all the others sounds that one can find, them, in System, and can be used by the user for system beeps, etc.

Bonjour,

Try to put your custom sounds into ~/Library/Sounds (the Library of your home folder, not of the System). And to be sure, name their extension .aiff (with 2 f) (I sincerely hope that this doesn't make any difference, but who knows, nowadays...)

Regards.

SIP is System Integrity Protection and prevents you from altering System files, you can disable it by booting to your Recovery HD, click Utilities in the menubar and select Terminal, type in csrutil disable press Return. Quit Terminal reboot your Mac. You should now be able to add your .aiff files to the System/Library/Sounds folder. If you want to re-enble SIP then boot to your Recovery HD, open Terminal type in csrutil enable and press enter. Restart back to your Mac.

If you don't fancy doing that then just put it in your Users/<home>/Library/Sounds folder

You need to be an administrator to do this, are you logged in in an administrator account.

How did you unlock the Sounds folder.

Have you disabled SIP?