Chrome: 67.0.3387.0
VRCore: 1.13.185188193
Android N
Device: S8+

What steps will reproduce the problem?
(1) Enter VR browsing, note the sound level of the button hover and clicking. Increase volume with remote to hear them clearly.
(2) Go to http://permission.site/
(3) Click on the Microphone button.
(4) select allow to grant microphone access to this site.
(5) click and hover over some buttons
<-- sound on buttons is now pretty significantly lowered
(6) browse to a new page.

What is the expected result? Sound on buttons should remain the same level during or at least after microphone is in use.

What happens instead? Sounds on buttons and hover don't return to previous volume levels. 

Note that the sounds in Daydream Home are still at the correct level. If you go to Daydream Home by clicking the home button on the controller. Then if you return to VR browsing via the Chrome icon in Daydream Home, the volume levels are returned to the correct state.