Some questions about the Web Speech API

I have a local environment where I have set up SpeechSynthesis and it works.
But I would like to be able to change volume, Rate, etc.
My problem is that I can’t get it to change any of this once it starts reading.
Have tried google and read here on stackoverflow but can’t find any answer as to whether it is possible.

And then my last question.

I have been told by some now that it should be possible to get it to use a custom voice possibly through a third party service.
But I haven’t been able to find any documentation on this and when I ask the same people again, they can’t find any documentation on it either.

So would also like to know if there are any of you who know the answer to this.