hAP ac² model has no speaker. can I add a speaker to this model myself to get sound?
if possible, what voltage is needed for the speaker?
Are there any proven speaker models with aliexpress?

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You could add an raspi-zero-pi to the usb-port, configure it as an usb-ip-node and a syslog-receiver.
Then ... for every event you need a sound ... you generate a syslog-message, which you can translate via 'logwatch' or something similiar into a special noise.
Very soon you have grown youself a respectible duck-pond !
(howtos for sound engine of the raspi-zero is avail in the forums over there).

You could add an raspi-zero-pi to the usb-port...

can you just solder the speaker to the board? Why should I rassberry if there is an easier way?
I think I need to use a simple passive microphone, but I'm not sure, I need help to find out for sure.

can you just solder the speaker to the board?

... not me ... you could do it yourself. But what is highly questionable: can that lead to a fruitful result ?
Odds that you ruin the board with such shenanigans are quite high.
Why not selling the board and have a beeping one instead.
Interchanging non-beeping boards into beeping boards may be also not the main pupose of this very forum (whilst having a beep is a legit wanting on gods earth ... no question).
So far, I have no idea how to identify a point on the hardwarebord where a beeper could brought to duty. Guess plans of the board-layout are somewhere on multiple harddisks, but I doubt the owners of these disks are willing to send you a copy (not only because they are not fully briefed about your soldering skills). But ... we never know !
Let's just wait, what's happening further !

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no predefined place on this board for a buzzer ... looks bad for a beeping hAP ac^2
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like on https://i.mt.lv/cdn/rb_images/1533_hi_res.png

no predefined place on this board for a buzzer ...

you seem to be right, there is no place to solder the beeper :-(