If this refers to a built in capability to use voice, Vivaldi won't ever do that as it would require an agreement to use Google voice services.
If this is a feature built in to the Google search and translate Web pages, it should be possible. If it doesn't work, then that is a bug.
@Pesala True. But that would imply that Vivaldi would have to be "always listening", which might be badly interpreted. (I trust Vivaldi. Dont get me wrong). Maybe a long press on the speed dial icon or smt ? Or, if that's even possible, bind it to google assistant.
I went looking for answers on the Tracker Software forum, and found that the PDF-XChange read-aloud feature for selected text uses the libraries provided by Windows.
Only Common Languages are Supported, all you need to do is download and install a suitable language pack.
One can choose between UK and US English. Some have a male voice, while others use a female voice. Pronunciation of unrecognised words is poor. PDF-XChange has controls for speed, pitch, and volume.
PDF-XChange Speech Options.png