CONCORD — Strings break. It is a professional hazard that violinists, violists, and cellists confront each time they go onstage. On Sunday afternoon, Steven Ansell and his three colleagues had barely launched into the first piece on their ambitious chamber music program, the world premiere of “Concordance,” a piano quartet by Yehudi Wyner, when one of the strings on Ansell’s viola snapped.

“Let’s start over,” said Wyner from the keyboard. And after a brief pause for string replacement, that’s what they did. A pleasantly unscripted moment, it gave the capacity audience a welcome opportunity to hear again the opening of an eloquently contemplative new work by one of Boston’s most productive and visible musical citizens.