Ceiling Exhaust Fan -How loud is .5 sones?

Is your attic finished? If not, I would mount the fan in the attic (not in the ceiling of your listening room) on top of the joists - utilizing good damping material between the fan and the joists (do a search for after-market noise isolation mounting materials/products). You can easily construct a duct between the fan and the ceiling of your listening room -which can also be damped (provided that you don't obstruct air flow). The duct work must be air-tight to insure maximum ventilation – I would suggest avoiding metal/tin duct work, as it is noisy (MDF may be a better solution. Make sure you damp the duct work from the fan, as the ductwork can act like a speaker cabinet – projecting the sound back into your room. This will not eliminate all of the mechanical noise, but it will reduce the noise compared to mounting it directly in your ceiling.

Note: you can also tweak the fan itself with damping materials (i.e.: soft fan/motor mounts, silicone motor mounts, etc.). The thing is, you want to decouple the fan from the mounting area (in this case your house).

Got talked into replacing two bathroom exhaust fans for a good friend and customer some years ago - half way thru the project I understood why his regular handeyman had passed on it.My buddy wanted less racket in the bathrooms of his buisness - found the quietest rated fans available about 6 or 7 years ago.After mucho struggle installed them.Results were: One fan was somewhat quieter than the old unit I replaced - very definitely not inaudible. Second unit was basically pretty much as noisy as when I started because most of the racket was coming from tin vent pipes banging around -couldn't get at these (120+ year old building)- so no joy.
Based on this very limited experience - I'd suggest that none of the "bathroom" fans are going to be quit enough for you. Remote mount the fan and use the flexible "plastic" HVAC duct to get to your room - while I'm sure you can find really quit fans ( try contacting folks who build recording studios)- I think it'll be a "ma hand me the nitro" moment when you see how expensive they are.

Nasaman:
I will offer my thoughts from a different perspective. I have had very extensive experience with Panasonic fans. We currently have five of them in our home. Bottom line, they are 'spooky quiet'.

The largest unit that we have, the FV20VQ3, which is rated at 190cfm and requires 6-inch ducting, is in-audible from ~10 few away, and is barely audible standing directly underneath it.

Sorry I cant answer your question directly, but this is real world experience. I have chosen to install all of them with timers as it is SO VERY easy to mistakenly leave them on because they are so quiet.

I installed the panasonic whisper stink fans in my bathrooms and put a 150cfm one in the ceiling of my HT room just behind my head.
I often will run it to evacuate smells coming from the kitchen while watching TV. Its barely noticable. The panasonic whispers are worth the extra money.

I also purchase large fans for hvac air scrubbers I dersign and have a sound calculation tool in front of me from LorenCook fan suppliers.
Sones .3-.5 = 14.5-15.5 dbs