Take a look at your Control Panel; navagate through the "Sounds"... at the playback tab, you'll see the installed audio devices..
If you have more than one speaker on the list, you need to select one working audio as default.... As you should know that "Speakers" on the very top of the list should already set as default, watch the speaker's output ( the green bar), while playing any youtubes, you should see the activity on "Speakers" bar, and if you do, it's very good chance that you muted it via Volume Mixer.
If you do not see activities next to "Speakers", let us know!