It’s a little disingenuous so say that “Vivaldi is closed source” and leave it at that. The vast majority of their browser is built on open-source code:
https://vivaldi.com/blog/technology/why-isnt-vivaldi-browser-open-source/
Only a small portion (~5%) of their bespoke UI code is closed. The vast majority of their source is published.
Obviously whether the small portion of the code they withhold is important to you is a matter of your own to decide, but I feel this was important to clarify.
If I heard this voice randomly somewhere it would be very unremarkable and I wouldn’t note anything unusual about it. It wouldn’t stand out as different to me.