Sometimes restarting Mac can restore order if things are not working as expected:
Apple Menu > Restart : wait for Mac to restart, and then log back in.
(Eg: Deletes temporary files, re-establishes connections, may update recent changes)
Re: I still can't understand why VoiceOver speaks so fast no matter which speed I set it to.
Wondering which settings you are using to set the speed:
Eg: Try checking : Apple Menu > System Settings > Accessibility > Spoken Content
You may need to experiment with adjusting the slider gently in small increments:
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Re: "I'm also unable to add apps to allow them to use personal voice as nothing happens when I click "+" "
You may use personal voice with Live Speech already ... seems it's the only Apple app.
To have apps available that you could add: You would need to have downloaded some third-party apps such as Augmentative and Alternative Communication (AAC) apps.
To allow those apps to use the Microphone (and maybe Camera too ?)
Apple menu > System Settings > Privacy and Security > Microphone : turn on
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Also: Those third-party apps may need to request ("ask"), or be given permission (allow") in Safari to use your Mac's microphone (and maybe camera too?):
Eg: Safari > Settings > Websites > Microphone (also : same with Camera if needed)