It actually isn't anymore, hasn't been for a few years. There used to be a requirement to disclose any material fact that may influence an Underwriter decision, but that has now changed to the opposite of what you've said; the onus is on the Insurer now to ask the right question, and if the right question isn't asked, the customer doesn't need to disclose. Basically means most FCA/FOS complaints go the way of the consumer for non disclosure these days.
I don't work in motor insurance, more leisure and household, but unless you class your band as your job, I can't imagine a need to disclose it. Come to think of it, I cant really think of an incident that may happen that would be unique to a music related venture that would be insured anyway. Is there an example?