The Fel Imp voices sound too much like the idea of what those young kids bots are in Transformers 2. (You know the ones I mean, the astonishingly annoying ones)
It's a horrible idea for some comic relief in my opinion.
Now you may not agree with me here, opinions are ment to differ after all. So in an ideal world, you shouldn't even agree with me.
But, take into account, that as a Warlock, you'll not be hearing your demon's speeches just once or twice. You'll be hearing it a lot.
And especially if the Imp is your demon of choice and you want it's upgraded version for the added 'oompf'.
Then you'll be hearing your little 10yearold buddy that just discovered the internet, or girls, a LOT.
Add to that, that these are static soundfiles, and don't automatically change over time. So we'll be hearing the same memes for quite a while.
That said, most WoW players aren't completely analphabetic when it comes to the internet and the things that play on it.
So it is pretty assumable that many of its players are already flooded with hearing and seeing memes everywhere.
Hence I don't see including memes in demon speeches as a thing that will be a succes.