The way this works is relatively straight forward when we do a step-by-step examination of how the rules initiate combat. So our PC states their intent to cast dissonant whispers on the unsuspecting NPC.
This is the first step for initiating combat. Surprise is determined like so:
The DM determines who might be surprised. If neither side ...
...a dagger deals 1d4 damage, and these types of players rarely extend
the logic to say "I stabbed that 13th level fighter with a knife, why
isn't he dead?".
This is a common and basic misunderstanding of how HPs are/were intended to represent the combat stamina of a creature.
Originally, they were a design compromise1 in Gygax & Arneson's &...