#Yes, the creatures are affected if the player aimed at the right spot regardless (e.g. 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 which the DM could had handled differently, knowing their location is not required; luck could have 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 creatures he is not aware are there cannot be targeted (unless he tries an area randomly within the *slow* spell area).

The answers on this question can give you more details: [Can I cast Slow on an invisible target?][1]

Another proof this works (thanks V2Blast) is the [Invalid Spell Targets][2] optional rule in the Spellcasting section of the *Xanathar's Guide to Everything* (p. 85):

> This section expands on the spellcasting rules presented in the *Player's Handbook* and the *Dungeon Master's Guide*, providing clarifications and new options.
> 
> [...]
> 
> ###Invalid Spell Targets
> 
> A spell specifies what a caster can target with it: any type of creature, a creature of a certain type (humanoid or beast, for instance), an object, an area, the caster, or something else. But what happens if a spell targets **something** that isn't a valid target? For example, someone might cast *charm person* on a creature believed to be a humanoid, not knowing that the target is in fact a vampire. If this issue comes up, handle it using the following rule.
> 
> If you cast a spell **on someone or something** that can't be affected by the spell, nothing happens to that target, but if you used a spell slot to cast the spell, the slot is still expended. If the spell normally has no effect on a target that succeeds on a saving throw, the invalid target appears to have succeeded on its saving throw, even though it didn't attempt one (giving no hint that the creature is in fact an invalid target). Otherwise, you perceive that the spell did nothing to the target.

So per this optional rule, you can target the area in a 5-foot square that you believe a creature would be and try at your own risk.

See the Question: [Do you need line of sight to cast spells on someone?][3] for more details. Especially [Christian Griffith's Answer][4] since it quotes Jeremy Crawford who is the official rules designer for 5e.


  [1]: https://rpg.stackexchange.com/a/126136/51979
  [2]: https://www.dndbeyond.com/sources/xgte/dungeon-masters-tools#InvalidSpellTargets
  [3]: https://rpg.stackexchange.com/questions/62840/do-you-need-line-of-sight-to-cast-spells-on-someone
  [4]: https://rpg.stackexchange.com/a/130885/51979