I don't watch much on youtube, but I have encountered similar strange stuff where audio is simply cut out, even the dialogue. I assume it's because some song was playing.
It's most likely because of rights issues. Perhaps the rights holder of that song only licensed its use within China, and not outside of China. So by putting it up on youtube they are breaching that license, thus the makers of the shorts and dramas simply cut it out or in this case play over it with something else.