No, this is not an appropriate venue. People pay money to ride on Metro and they have a right to expect "relative" quiet on the ride. There's a reason you have to use headphones when you listen to music on metro - so that others around you aren't subjected to your choice in music.
Just another turd in the punch bowl
Of course it's all right; this is one of the joys that make city living so interesting. Metro is not and never has been a guaranteed quiet ride.
Just like most folks don't like to hear music blaring from someone's phone with a bunch of curse words and nastiness (see "Do Me" music that's all over the radio now), there are a good number of people who don't subscribe to Christianity and don't want to hear church music being sung at them on their way home from work. That doesn't make city living interesting. That just makes it a little more annoying.
If I wanted to hear it, I'd go to church. Leave me alone during my commute.
If I wanted to start belting out some "Iron Maiden" or "Black Sabbath" during a packed train full of commuters would _that_ be OK? Of course not, and neither is this. That being said, I'm not sure that much can be done about it. While playing music w/o earphones is prohibited, I'm not aware of any law/regulation that prohibits singing.
just another invasion of privacy from busybodies who wont leave you alone. the proselytising urge has nothing benevolent in it. it's just an attempt to push a church on you, no less offensive than a commercial sales pitch.