There may have been some cost considerations during the initial installation. Now that you have some credibility with them, you could explore the "sounds OK" part.

I never try to tell anyone what sounds "best", I always ask. If the customer is not able to discriminate or doesn't care, I leave things alone. In this case I would open the discussion by asking if they would like to have better control control over the system. (Allowing different music here'n there while saving trips to the speaker control boxes) Initially, pull one pair of speakers off the selector and let them live with that. If there is a sudden revelation that "this room sounds much better than the others", then you can have a more thorough discussion. While it might chaff some of us, many customers simply want background noise and they are fully satisfied with earbud quality.