According to the Waterville Opera House website, the rules that Bela Fleck and Abigail Washburn record by are strict. That is, if taking the sound of a banjo to where, as Fleck said, “We’re expressing different emotions through past techniques and going to deeper places” can qualify as strict. The duo performed Thursday. One member of the audience, a toddler, decided to join in, perhaps screaming a tad out of key, inspired by the musicians’ Echo in the Valley. Echo in the Valley is their follow up to their acclaimed, self-titled debut that earned the 2016 Grammy for Best Folk Album..