Hmm, I assume when you click the speaker icon on the YouTube player controls that the slider is set to have some non-zero amount of volume. Does Firefox display a little speaker icon on the page's tab up on the tab bar indicating that Firefox is detecting audio playing in the page? If Firefox displays the audio icon, does clicking it make any difference? It operates as a mute/unmute button. If Firefox thinks it is sending audio to Windows, could you check the mixer? 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. (''I don't remember whether Windows 8.1 has this in the same location as Windows 7 and Windows 10...'') Here you can see whether Firefox shows any volume or is muted at the system level. Anything unusual? YouTube usually uses the built-in HTML5 video player. However, if the site uses Flash, check for a muted Flash player in the mixer. When Protected Mode is enabled, the mixer usually would show that Firefox is NOT sending audio, that the audio is coming from a separate Flash process. <img src="https://support.cdn.mozilla.net/media/uploads/images/2016-07-10-12-15-12-df19e1.png" width="500">