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I need to ask first of all, what Apple computer or other equipment you are asking about - a MacBook or Apple Air laptop, or an iPod, or an iPod dock, or... what, please?
There are, depending upon what equipment is having the problem in the left speaker or left channel, different things to try doing.
So it's the built-in speakers on the Mac Pro that have the problem? not external speakers?
Thanks for the video, I'll view it in a moment.
But is it clear, that your MacBook Pro has no external speakers, just the built-in ones that are part of the body of the laptop in question - right?
If it has external speakers hooked up to it, unplug them, try again, see if the crackling persists...
That is a severe problem with your laptop's sound card, I think.
The crackling noise... is hard to figure out, but basically an electronics problem with the sound card almost indubitably.
It does no respond to being put on Mute, nor changes to Volume setting... so, it's a 'breakage' in the sound card. There's not much else to say to it. Except: how old is the MacBook in question, please?
(You can try a reinstall of the sound card drivers. That's advisable before deciding on repair or not, etc. But I somewhat doubt that will help.)
So what is the crackling caused by exactly? maybe it's voltage surges in the sound card (in which case volume controls wouldn't affect it.)
But I'm sorry to say, esp. if the sound appears even before the computer has fully started up and loaded the OS, it may be the sound card is defective.
But when *does* the crackling start, in the course of booting up? it might still be a corrupted driver for the sound, or the like.
That sounds very like a corrupted driver (though it could, still, be the sound card hardware at fault.)
1: what model is the MacBook, please? so that I can identify the sound card driver.
2: what MAC OS X version is on it?