The notification sound is unimaginably loud. It appears to be playing on the same channel and at the same priority as the instance queue notification (also unimaginably loud but that's not your problem). Would it be possible to move this to the normal SFX channel level so that it could be affected by the volume slider?

It is on the master volume channel, same as dungeon queue and that is the slider you want to adjust. It was on the standard one before but caused issues. Majority of the addons will use the master channel as that way you can disable wow sounds and still utilize the addons sounds and causes less issues for some.

You can now make ATT play audio on the "sound" channel rather than on the "master" channel by using a toggle in the Interface options.