Chiming in to add to @cameo43 – Pep Band is certainly small but growing and they do a marching show each fall so they are technically also our marching band. They play in the stands for basketball and hockey (sometimes) and you get PE credit for participating in either the marching show or playing stands.
For classical, there’s the orchestra which is a string ensemble in which everyone can participate, and wind players can audition for the limited wind, brass, and percussion seats. There is also a concert band in which all wind players can participate, and an audition-based brass ensemble. There is a string quartet which you can audition for and carries a modest scholarship and professional gigs. We also sometimes, based on interest and instrumentation, have a flute choir and wind quintet but those are mostly for fun.
There are (at last count) three jazz groups on campus–jazz lab (for fun/club), jazz ensemble (selective combo), and stage band (“big band”). All but jazz lab are audition based.
For academic classes that would be part of your regular course load, i.e. for humanities, there are certainly music theory courses like fundamentals and orchestration, but also music history, world music (ethnomusicography), and music technology including music robotics.
Be sure to ask about private lessons! WPI will connect you with a local teacher. There is an extra charge paid directly to the teacher. You can do one term of lessons as a 1/3 independent study.
As for course load, ensemble participation is a 1/6 credit per semester, and you can take up to a full year for credit. You can pair it for a semester with a regular 3/3 + 3/3 course load plus a gym credit and not technically have a full overload, just 3/12 beyond your 6/3. (Yes, the fraction system confuses me still.) The A Term musical is usually offered as a 1/3 independent study credit but you can only do that once.
Also, I’m a music minor here and those are super easy to get. If you do your humanities concentration in music, you can double count three 1/3 courses, then take an additional 2 x 1/3 courses plus a minor capstone (for me a recital).