Moderator's Note: Changed the topic title to be more specific (and visible in case he's on the site)
As a side note, using existing mixes is just another layer of effort in the process (instruments->songs->mixes->remixes). It's totally fine, I think where people have the issue is:
1) Not attributing any credit to the original mixers
2) The scale of the reuse. For instance I noticed CMU's mix from last spring was used wholly unedited from start to finish. That is fundamentally different then taking a particular song you thought was mixed well.
Neither point is particularly offensive, but both together is what is irking people I think. The truth is it's not worth any effort or frustration, so if you hear something, simply leave a polite and professional comment on the youtube video and that really should be enough.