I noticed that happening too. It doesn't usually bother me because I can still (just) hear the notification when I'm navigating other tabs.
If you have a look at this web page
http://www.reddit.com/r/chrome/comments/200swd/, it suggests the reason may be because the 'search by voice' has been activated. So when you open another web page, the computer expects you might try to give it a voice command to google and so reduces the output of the spekers, presumably to help prevent feedback.
This website,
http://www.askvg.com/fix-windows-7-automatically-reduces-system-volume-for-games-pc-calls/, suggests the following fix :-
1. Open "Sound" applet from Control Panel. You can also access it directly using "mmsys.cpl" command in RUN or Start Menu searchbox. Or right-click on Speaker icon present in system tray and select "Playback devices" or "Recording devices" or "Sounds" option.
2. Now go to "Communications" tab and check the option "Do nothing" and Apply it.
You can't access the 'communications' tab from the speaker icon in the system tray, so it's important you do it as suggested above.
Cotton buds might work too. though