Greetings, Bandit!
Yes, I sing with both the Voices of Baha and the Baha'i Gospel touring choirs, as well as the local Baha'i chorale here in the DC area. :-)
And both choirs routinely sing "Amazing Grace" as part of their repertoires!
As to the restriction on instruments at the seven (soon to be eight) Baha'i Houses of Worship, this applies only to the main temple area. In the downstairs meeting rooms, instruments are welcome.
We sing lots of different sorts of music, including some Christian music (especially spirituals and gospel music); in addition, we have plenty of uniquely Baha'i music by various composers, including Baha'i gospel music and several classical styles!
I hope you get to hear us someday! :-)
And in the meantime, you can at least hear excerpts of many of our songs at:
www.global-music.org
BTW, as to worship services, various Baha'i communities hold these as well as prayer meetings (these are separate from our Nineteen-day Feasts, and the public is warmly invited!). In addition, the Baha'i scriptures say that work--our professions--done in a spirit of service to humanity--ranks as worship of God!
Best,
Bruce