A little too unbelievable for me I love TAGS. But this is not one of the better episodes. I am not too concerned about the morality of Barney being made to look foolish. This is a TV sitcom so there's no point in getting too worked up over something like that. The reason I don't like it? Simply because NO ONE, not even Barney, would be so easily duped into thinking that he's singing into a "solo" microphone and that he barely has to whisper in order for his voice to be heard loud and clear. Golly gee, I don't think even The Beaver would believe something like that. :)
Unless the real singer behind the curtain is in perfect synchronization with Barney it should be obvious to Barney that he is not hearing his voice. Not to mention that it doesn't sound anything like Barney's voice. This premise is just so unbelievable that it takes away from some of the funnier parts of the episode.
Also, there's another part of the plot that is a little shaky for me. If they already have this singer why is everyone so worked up about the original soloist not being able to perform? Just have this guy take his place. You would think that John Masters, the choir director, would know that this singer is already in the choir and just use him. They would not even get to the point of having to ask Barney to sing. They would not have to go through the trouble of trying to fool Barney. Or go through the trouble of moving the practice location and keeping it a secret from Barney. Just a bit too unbelievable for me.