The morning talk show co-host was using public transpo Tuesday when a guy busted out a rousing rendition of "Stand by Me" ... so he started recording. The presumed host of the 'A.I.' reboot didn't seem to know all the words, but still enjoyed the show.

Ryan got a gentle reminder at the end, though -- this ain't an 'Idol' audition ... and filmed performances ain't free.

He should be able to afford it ... he's expected to bank $10-15 million to join the 'A.I.' team.