sudden humming sound and loss of video audio

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Last week I experienced a peculiarity that I can't think of any reason
why and how did it happen...
I was playing a video and while it was running, I plugged in an
external hard drive, which immediately changed the sound of the audio;
from normal to suddenly being drowned by a persistent humming sound.
I closed the video and played the music files by windows media player.
To my surprise what comes out was the persistent humming sound.
I shut down the Pc and restarted it, and still the same humming sound
I heard from it, from my music files and video files.
Instinctively I went to the system restore in control panel and
restored the PC to an earlier date (about a week before) and then
everything went normal...
Until now I am still perplexed and still wondering what is the root
cause of that humming/ buzzing sound that replaced the audio in my
music and video files being played?
Any ideas why and how did it happen

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