Learn French: Tous les tambours que nous avons entendu battre étaient énormes. - All the drums we heard being beaten were enormous.

Normally, a past participle used with avoir agrees with a preceding direct object. However, for verbs of perception (like entendre or voir) followed by an infinitive, you only make the agreement if the object is actually performing the action of the infinitive. Since the drums (les tambours) are not doing the beating, entendu remains invariable and does not get an -s.

Why is the active verb battre used here when the English translation says "being beaten"?

In French, when an infinitive follows a verb of perception, you always use the active form of the infinitive, even if the meaning is passive. French avoids using a passive construction like être battus in this specific structure.

How would the grammar change if we heard drummers playing, rather than drums being beaten?

If the direct object is actually performing the action of the infinitive, the past participle does agree! For example, "The musicians we heard playing" would be translated as "Les musiciens que nous avons entendus jouer". Because the musicians are the ones doing the playing, entendus gets an -s to agree with les musiciens.

Why does the sentence use the passé composé (avons entendu) for one verb, but the imparfait (étaient) for the other?