I would definitely say that Brandeis is the most politically active of the three. It was literally founded on social justice. Brandeis is certainly predominantly liberal, but I wouldn’t call it hopelessly PC (though that’s somewhat relative). I think the social justice element also fosters a collaborative and non-competitive atmosphere - when I toured there, the main thing students I spoke to talked about was how kind and supportive everyone was.
I think all three of them have good opportunities for music, but in different ways. Rochester has Eastman and Case is connected with CIM, but as these are world-class conservatories, it might be harder for a more casual player to take advantage of these connections. Brandeis, while not affiliated with a conservatory, appears to have a strong music department - Leonard Bernstein once oversaw it. I spoke to someone who had been actively involved with music as an undergrad in a variety of ways and was very happy with his experience. A benefit of this kind of music department is that your kid wouldn’t have to compete with conservatory students for teachers/ensemble seats/etc (though I believe Case and Rochester both have ensembles separate from the conservatories as well).
Disclaimer: I have only visited Brandeis. I just noticed your first sentence - sorry! Hope my comments are still helpful.