Or HaChaim on Deuteronomy 5:24/אור החיים על דברים ה׳:כ״ד

1 ושמענו ועשינו, "when we hear (them) we will carry (them) out." The reason the word ושמענו is introduced by the conjunctive letter ו is that the people had asked Moses to be their intermediary when receiving G'd's commandments. They explained that initially, in Exodus 19, 9 they had asked to hear the words from the teacher i.e. G'd Himself, instead of merely from the pupil, i.e. Moses. However, in light of their deathly fear they reversed themselves concerning who they were willing to hear the words from. They now made Moses their intermediary. The letter ו in the word ושמענו is the allusion to their previous attitude in the matter. They added the word ועשינו "and we will do" to indicate that the manner of their מצוה performance would be as if they had heard the instructions from G'd Himself instead of merely from Moses, His intermediary.