Stop a looped sound when player stops using entity

[Sam Behner]

So when you hold 'E' on this entity it plays and sound and gives you armor, for some reason I can't figure out how to make it stop playing the sound when you let go of the 'E' key. Here's the code I have
local soundPlayed = false; -- To make sure it doesn't play the sound more than once
function ENT:Use(a, c)
if self:GetAvailArmour() > 0 then
local curArmor = c:Armor();
c:SetArmor(curArmor + 1);
self:SetAvailArmour(self:GetAvailArmour() - 1);
-- print(self:GetAvailArmour())
if not soundPlayed then
self:EmitSound("items/medcharge4.wav", 75);
soundPlayed = true;
end;
end;
-- I would assume I put something here because it's the end of the use, wouldn't work though
end;

[DaddyFrosty]

sound.Add
and
Entity/StopSound

Add the "items/medcharge4.wav" with sound.Add, when player stops holding E do ENT:StopSound("name you assigned with sound.Add")

[Sam Behner]

I know how to stop the sound, I don't know when the player stops holding 'E'