A question was asked on Quora, “Why would some one start talking out loud while I am performing on the piano?” With the performer explaining that during an intimate performance with friends, one new friend would talk and interrupt. Here’s my suggestion:

The easiest way to find out why is to ask and set a boundary. “You may not be aware of the etiquette for this situation, so I just wanted to let you know usually people listen during a performance without talking. Is that possible?”
Listen to to the response carefully. Be clear about your own options. “That’s great, thanks for telling me that. Next time just please wait until a better time to talk.” Repeat as much as is needed
And if it’s clear “If it’s not for you, next time I’d prefer that we spend time together doing other things. Thanks!!

Sponsor: Love typography? Typeface Trivia Playing Cards: 52 cards each showcasing a distinct favorite typeface