if playing with the volume buttons isn't getting anywhere - try a reboot or restart.
is there a little 'speaker' icon anywhere on your controls ..?

Easy to do.

If you were, is there now a diagonal line through the speaker icon at the bottom left of the video?

If that's the case, either click on the speaker icon itself or hit 'm' on your keyboard.

The above also works if you were watching a youtube video on DU.

Otherwise, can you say what other software you were using?

Open Settings and go to System > Sound.
Under the Advanced section, click on Volume mixer.
Click the Reset button to restore the original volume settings for all apps.

Alternate methods
Right-click the speaker icon in the system tray at the bottom-right corner of the screen.
Select "Open Volume Mixer" from the context menu that appears.
Adjust the volume levels for different applications and system sounds as desired.

Your "alternate method" did the trick.

Again, thanks!