Could you check the Windows Volume Mixer to see what application is sending the sound?

On the Windows Taskbar, in the "notification area" at the lower right (sometimes called the system tray), right-click the speaker icon and choose Open Volume Mixer. Usually the bar for Firefox will be moving if the audio is coming from Firefox. If the site is using Flash, and Protected Mode is enabled (the default setting), the mixer usually would show that Firefox is NOT sending audio, that the audio is coming from a separate Flash process, but it could be playing in a Firefox tab.

Did any of the windows/tabs have a player running?

Other possibility:   the narrator has accidently been turned on   (happens sometimes during a Microsoft update).
Go to Settings > Ease of Access > Narrator - and turn it off.