why do a lot of old music videos on youtube sound remixed and not the original

Part of it is probably due to digital data compression, which is a lossy process. For example, MP3 compression throws away sounds that are imperceptible or almost imperceptible, and sometimes you hear a sort of "crackle" due to it.

Another part may be that soft passages of the music are often made louder because that "sounds better" to modern ears (especially in a car or other place with background noise). It's called dynamic range compression, and no remix is needed; it can be done on the fly as part of the streaming. I don't know how often dynamic range compression is applied to a stream, but that could explain what you're hearing.

I think it goes on the views of the video.

Maybe people posting "their" version of the original?