If the movie heavily relies on rock music, then they can just have a rock band play the live music instead of an orchestra. Or maybe it could be some members of the orchestra may also be rock players, if both types of music are in the movie.

Actually, that's just how movies used to be.

We figured out how to record and play film long before we figured out how to record and play music/dialogue in sync with the movie. In the age of silent cinemas, it used to be that theatres would have an orchestra playing live music to accompany the film.

This is selection bias. Just because you don't hear it doesn't mean that it doesn't happen.

We only hear about diseases with land animals because that's the diseases that we do researches on. We do researches on them because they are the animals that are important to us, one way or another.

Diseases on animals that are similar to us affects us directly because of cross species infection. Also, we don't tend to hear or care about animal diseases unless it's economically important. Land animals are farmed at a much greater scale, so we care about them more.

Diseases with sea animals happen aplenty, we just don't know or care enough about them to do the research needed to actually notice them.

Simple as that.