נָ֥חָה שָׁקְטָ֖ה כָּל־הָאָ֑רֶץ פָּצְח֖וּ רִנָּֽה׃
The whole earth is at rest, and is quiet; they break forth into singing.
Rashi on Isaiah
opened [their mouth] in song This is the song they recited, “All the land rested, became tranquil.”
Ibn Ezra on Isaiah
פצחו They break forth. The root פצח in Arabic means to speak poetically.