[Updated] Is sound OK for you here?

[space-time]

OL, more testing. I reset PRAM, started the machine, played several songs in iTunes, sound is fine.

I play this damn video, sound is bad (like a scratched LP). I go back to iTunes, now sound is bad.

Something is definitely messing up my sound.

EDIT: but after a few seconds, iTunes recovers and sound is fine.

Now testing an iPhone...