At about 28 minutes, there is a singer that sings the song Ruby

LVVd12Yl0XM

Link in case the embed doesn't work https://www.youtube.com/watch?v=LVVd12Yl0XM

I checked the sites that usually list cast members but they are not listed.

Seems to be the same singer from the New Year's Eve Party episode (not Gordon Polk)

You'd think this would be easy to solve, but it's not.
I've been researching this for years now.
If he was a solo performer from the time, he's probably be known by now.
So he could have been one of the many vocalists that were part of the band then. He looks mid 20's, so I'm thinking he wasn't with the band long at the time. Could have lasted years after, though.
I have looked through 100s of photos & found nothing.
The only update is that the actual song title is "Ruby (It's You)"
Here are the lyrics....
http://www.theguitarguy.com/rubyitsy.htm

https://en.wikipedia.org/wiki/The_Dorsey_Brothers#/media/File:Dorsey_Brothers_Orchestra_1934.jpg

Is he in this picture maybe?

That pic is the only good pic from the 50's i've seen. It doesn't look like he's in it.

He remains a mystery man.

I passed by this thread while trying to find the answer to the same question, and I think I figured it out! The male vocalist that appears in both The Honeymooners: Lost Episodes S1, Ep34 The Dorsey Brothers Show, and S2, Ep11 New Year's Eve Party, alongside Tommy and Jimmy Dorsey, looks a lot like Johnny Amoroso! Amoroso was a young trumpet player and crooner with Tommy Dorsey's band in the early 1950s. If you look closely in the New Year's Eve episode, when the vocalist walks away from the mic, he picks up his trumpet!

Just wanted to stop by here again and post the answer in case anyone else is trying to figure out who the mysterious singer was in those episodes. I know the majority of the big band vocalists from that era, so when I saw one I didn't recognize, it drove me crazy!

https://i.imgur.com/GyfvYdp.png

That sure does look like him. Thank you!!!