What else do you expect though? You launched a long running task. It is done. Microwave goes ding.

My expectation is described in the comment you replied to and side comments

Microwave's ding is also bad UI, especially when it's repetitive as it's just useless noise. But even the first ding - what value does it have if you already have exactly the same notification category - audio cue - built right there in the process: the machine noise disappears when it stops working.