[Trisquel-users] Trouble with sound

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A few days ago I was watching a YouTube video and adjusted the volume with  
the hardware switch on my laptop. Abruptly the video's sound was replaced  
with a high-pitched throbbing squeal. I determined that this wasn't a problem  
with the video, because it occurred with other YouTube videos and a video I  
had saved.

Restarting the computer fixed the problem; however, I'd like to know what  
might have caused it so I can prevent it from happening again.