I am a musician, a brass instrumentalist at a professional level. I have no opportunity to serve in music ministry in local parishes, except on a rare, not even annual, special occasion. I received an invitation to serve at a local non-denominational Bible church with a very high quality music ministry. They ask me to serve one to two weekends a month. It has been a great blessing so far. Is it wrong to serve in a non-Catholic church when my own Church says it doesn’t need my gift?
Had there not been that hint of bitterness toward the Church at what you apparently perceive is its refusal of your musical gifts, I would have said that there ordinarily is no problem with occasionally participating in non-Catholic Christian music ministry. But in your case, there is a problem because of your attitude toward ministry in the Church.
I suspect that you have not received many invitations to participate in music ministry at Catholic churches because there ordinarily is little need for brass instruments at Mass. As wonderful as brass instruments are in jazz ensembles and marching bands, that kind of music is not appropriate for Mass. Some parishes do have contemporary choirs that use guitars and drums, and those parishes may therefore be able to find a spot for a brass instrument, so you may want to do some looking around at parishes in your diocese for such opportunities. But if they are not forthcoming, I urge you not to conclude that the Church is spurning your gift.
Any ministry in the Church should be considered a gift from God and not something to which anyone has a “right,” no matter how talented they may be or how eager they are to share their talent. Try to remember that we are called to serve, and that when our desire to make public our talents becomes more important to us than humbly serving the Church should it need us (and accepting when it does not), then we are no longer serving but showing off.
This doesn’t mean that God doesn’t want you to share your musical abilities. You’ll just have to become more creative in finding legitimate opportunities to share them. Perhaps you might approach Catholic conferences for both youth and adults and offer them your services. Perhaps you might visit local Catholic schools and nursing homes to educate youth in the importance of music and to entertain and uplift the elderly. But I do urge you not to allow your disappointment at not being able to play at Mass to lead you out of the Church.