I’ve heard that some priests recite the words of absolution while the penitent says the act of contrition. Perhaps to save time, especially in larger parishes.
You don’t need to hear the words for the absolution to be valid; heck, you can be unconscious. What matters is whether he said them, specifically “I absolve you from your sins” or some other faithful translation of the original Latin. Not hearing them just means you can’t be certain he said them right (or at all).
You do have the right to ask the priest to pronounce the words of the absolution if you’re uncertain if he did, or if he did so using the wrong words (e.g., “You are absolved of your sins” or “May God absolve you of your sins”). Next time I would just do that if you’re uncertain.