Okay, so you're trying to sing and the guitarist next to you is too loud, too inaccurate.

1. Don't say anything that makes him look bad and you look good. Make it seem like you are the weak one. Say, "Brad, my voice is strained today, and your playing is a bit too loud for it. Would you please play softer?"

or 2. Stand up and walk further away, still singing. This will embarass the guy, but it's not as bad as humiliating him publicly.

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We would have fewer of these unharmonious guitarists if so-called experts would stop telling them that most songs can be accompanied by only 3 chords, the I IV and V.

PS Forgive me if that's not how you spell 'embarass.' However I type it, it looks wrong.