The book of the covenant — i.e., the book which he had written overnight, the collection of laws and promises which we have in Exodus 20:22 to Exodus 23:33.
In the audience of the people. — Heb., in the ears of the people.
And they said. — Having heard the ipsissima verba spoken by God to Moses, they repeated their previous acceptance (see Exodus 24:3), adding a general promise of obedience.