As an aside, I wouldn't have an issue with riders getting up to go to the water fountain--in fact, I encourage people in my classes that if they need to refill their water bottles at the water fountain, they should do so--it tells me that they are working hard and drinking the right amount of water.
There are some threads around about students using their ipods and listening to their own music--you might want to read those and see what approach you want to take.
Have you talked to other instructors at the facility about their experiences and their classes? Ask the riders for their music recommendations and put together a "people's choice" ride. If language is an issue, see if there are photos, graphs, charts, videos, etc., you can use to get your point across--it will probably get their attention.
I'd also say, at the end of the day, you already teach 6 classes a week: it's OK to walk away if it's not working out for you.