Because most machines these days have volume control built in. You can pause the talk, though, if you want it to shut up for a moment.
Thank you for the feedback. We just launched VR last week and are actively improving it. I will put a volume control in the Backlog(;
I was so annoyed I didn't even listen to the audio and immediately shut down the page.
For instance, if I have Gmail open in the background, there's no reason that adjusting the volume of whatever's in the foreground should also force me to change the volume of incoming chat notifications.