Why do they modulate

It could be many things, but most likely you're recording them too loud. You should turn the gain down on your recording interface.

But just look at the footage in the vid itself

The footage is cool, but you should also pay attention to the detail of the audio. Music is an important part of music videos as well as movies.

BTW, could you get Dark Wing Duck Prison Break back on youtube? That movie rocks!

What audio
I think the footage in the vid is the most important thing

What audio? The music and singing. Both audio and video are equally important.

darkwing duck would care more about the audio quality

Why would HE care about the audio quality , the quality of the footage looks PERFECT

Because he's also listening to the song. It's a MUSIC video. Music is half of it.