#Yes the creatures are affected if the player aimed at the right spot regardless if he knew exactly where they were because he heard them#
The Player heard them, aimed where he thought they were, so unless some Meta is involved wich the DM could had handled differently, knowing their location is not required, luck could had been on his side too.
From the Spell Slow:
You alter time around up to six creatures of your choice in a 40-foot cube within range
It is not required for them to be seen if we read the description, since D&D has a lot of rules the spell cannot contain all the exceptions, but creature he is not aware are there cannot be targeted.
The Answers on this question can give you more details.Can I cast Slow on an invisible target?