Whenever I use my "Marked for Death" shout, I'm detected. Is there any enchantment/perk/potion that will make the shout quiet, so I won't be detected? Or is this more of a "use when they see you" shout?

Use it when they see you. As far as I know, there are no quiet shouts. I've even been detected using Aura Whisper.

because when you SHOUT you are quiet. The only quiet shout is the throw voice shout.

Its called a "shout".

Well, Marked for Death is an offensive shout, so I don't get why you'd expect to remain hidden when you hit an opponent with it. That's like casting a really quiet Fireball at someone. There are some shouts that you can use and not be detected(Aura Whisper and Throw Voice at least), but this is not one of them.

***SPOILERS***

I just assumed that since the shout was from the dark brotherhood, and they're stealthy by nature, it'd be quiet. My mistake

Well, that makes more sense then.

. Location means nothing.

I usually only use it when I'm forced into a fight on my Bosmer assassin character. Thinks it's there to stealth characters out of a tight spot if they get caught, makes things a bit easier to kill.

Remember, the shout causes health degeneration. if you had no life you could shout the guy to death from a shadow if it was silent