There is a "mute sound on this tab" option, but it doesn't look like that's the solution.
See, I realized these sounds don't have anything to do with the site, not even with the browser - as I'm getting the same "boing" sound even right after the machine's been started up, when only the desktop is up. It's the same alert sound as when a usb stick is moved in and out of a port, but it seems to happen in an irregular way when the mouse is getting moved about , with the cursor (and the mouse is usb connected). At one point I actually got a quick error message saying "can't recognize the new USB device" but it disappeared before I had time to look into it further. Maybe the mouse is getting a bit old (been using it for at least five years)? But I checked both the mouse and all default USB disk places on the control panel, it says they're all functioning properly and I'm not having any real trouble with the mouse. I don't normally have any usb stick plugged into one of those ports as a steady thing.