"You have a quiet voice."
Translation:У тебя тихий голос.
You use «есть» when you want to make it clear whether there is something, but omit it if you describe qualities, possessed items and so on. Example:
«У него есть машина» — a simple statement that he indeed has a car.
«У него машина» — means that he has a car, not a boat or bus or I don't know what else.
You normally don't use «есть» when describing people because it sounds like "a quiet voice" is something physical, an item you can put on a shelf.
It seems that technically you are correct. What is the difference between being a native speaker and someone that learns formal language. Just one of those subtle nuances that you have to learn through usage. I made the same mistake and probably will several more times!
Вы is also the formal "you" - singular, addressed to someone to whom formal deference is owed, like your boss at work.
Perhaps it's a way of showing that you shouldn't be making personal comments to someone your should be treating formally.
Except I can imagine a host of situations where I'd be talking to someone I didn't know and thus would address formally, ask them to speak up because they had a quiet voice.
Abraham Lincoln had a thin, reedy voice which was ill-suited to public speaking in the 19th century. I can't imagine talking to him about how quiet his voice was without using the formal "you".
I've just read Norrius's explanation from a few years ago, which I think is pretty clear. If I've understood him well, you use есть to indicate ownership of an object, whereas the sentence without "есть" indicates having a quality or an attribute: У меня есть машина - I have a car (ownership of an object) У меня тихий голос - I have a quiet voice (an attribute of mine, and not an object I own)