When the priest absolves you in confession, he is acting as God’s mediator, so it is God himself who is really forgiving you. The fact that you didn’t quite hear the words of absolution still make this a valid confession if you had the proper attitude when you confessed your sins.
God, in His mercy and goodness is like the father in the story of the prodigal son, patiently waiting for the return of His long lost son (the sinner). When we sin (all humans by our fallen nature are prone to sin), all we have to do is repent right away (tell him we’re sorry) and we’re forgiven. It’s that simple. If the sin is a serious one however, just make sure to go to confession as soon as possible.
I believe all God is looking for is a repentant heart. Confession was made for man, not for God. God does not want you to agonize over this small scruple. Just believe in your heart that you are forgiven.
God bless