I have the same problem, Leeneia, although I hadn't traced it to a specific cause. It could well be YouTube videos. Gee, I'd hate to swear off them.
If it happens, all the sounds on my computer are crackly after that - MIDI and CD and MP3 recordings, and any other sounds. The one solution I've found is to replace the driver for my sound card, which is a bit of a scary operation if you're not savvy.
I'm assuming you have Windows XP or Vista and that you have an always-on broadband Internet connection:

Right-click on My Computer ("Computer" in Vista) and select "properties.

Then select Device Manager

Look down the list until you find the selection for "sound" (may have a little loudspeaker icon) - you may have to click on a "plus +" sign to get to the actual sound card driver

Highlight the sound card driver, and delete it - if it asks if you're sure, say "yes"

Reboot your computer, and the sound card driver will reinstall.

That should take care of it....except that now the headphone jack isn't working on my computer.

Good luck.
-Joe-