Yes, a little more info would be helpful. If you are in CA and planning to stay West Coast, it wouldn’t help to recommend WPI (Worcester, Massachusetts), which is a great STEM school and also has a great music program too.
Same with the financial side. Answers will be different if your annual contribution is $10k, $20k, or $70k.
In general, look at in-state publics first. They are often likely to be among your most affordable choices. Sometimes a school with a very high “sticker price” can be as affordable, or even more so, but much, much depends on test scores, financial side, etc.
Anyway, disregarding all that, WPI and Case Western Reserve (Cleveland) would be great choices if your test scores are also high. CWRU (“Crew”) has a relationship with Cleveland Institute of Music, a top music school, and a world-class concert hall (Severance Hall) immediately adjacent to campus, which students can attend free or subsidized. Both schools are extremely competitive, not insanely competitive, like MIT/Harvard/etc., but very competitive. These have $60k+ annual sticker prices, though they can both be fairly generous with merit/financial aid. Again, depends on the applicant.
Also, you can find both great math and great music at lots of schools. I’m most familiar with William & Mary. Top-notch academics, and I know several current students very involved in music groups and music theatre, one who lives in a house with others in their music group. So lots of opportunities out there.
Good luck! (And provide a little basic info on the financial side, location (regional preference, if any), large/medium/small, urban v. smaller town.)
Oh, and also try to get all of this done early over the holiday break. Applicants often do better with financial aid the earlier they apply.