You don't need to see them for this spell but you still need a clear path.
You have total cover from each other in this case and therefore do not have a clear path to the target (PHB, p. 204) to the target. Take a listen to the Dragon Talk podcast for 1/19/2017 (specifically starting on 32:14 for your issue). Jeremy Crawford goes into more detail on targeting and how it is supposed to work.
He also tweeted the following clarification while leaving it to an individual DM:
Q: Dissonant Whispers: is the first sentence good enough to ignore total
cover, or must the spells description mention it?
JC: Dissonant Whispers isn't meant to circumvent the need for a clear path
to the target, but hearing is the key part. DM's call.
Then the following tweet about the difference between visibility and cover: (emphasis mine)
Q: So "A clear path to the target" section on phb204 doesn't apply to
Slow spell? Or is it for the casting point?
JC: The clear-path rule is about there being a path clear of total cover.
It's not about visibility.
The clear-path rule as a definition is widely accepted (although it is ambiguously defined in the PHB) as you cannot have total cover between the caster and the target in a straight line.
Certain spells like Sacred Flame do not require a clear path to the target by design, as detailed by JC on that podcast, since it descends upon them from on high.
Comparisons and the exceptional.
From Sacred Flame:
...gains no benefit from cover for this saving throw.
No such text exists for Dissonant Whispers.
The general rule for all spells is that you require a clear path to the target. Dissonant Whispers does not countermand that general rule, therefore it doesn't remove that requirement. Unlike Sacred Flame which specifically says the target can't benefit from cover.
It could go something like this:
- You could cast the spell at your target then duck behind the cover.
- Then, of course, the enemy could easily use the Ready action to hit you
with Magic Missile as soon as he sees you.
- Your next turn you would have to peak over your cover to target him
again. Tactics in action.
You can target creatures you can't see with Dissonant Whispers but only if you have a clear path to it.
Credit to @Rubiksmoose for helping with the Tweets which are easier than going through the Podcast covering the same general material.