I am Chaotica (Chaos Rhapsody in-game), and this is my AD Sona guide. I had come across a few before and they didn't really work for me the way that I wanted to. My solution was to take what I thought would fix any issues I had with a standard ADC build and mesh it onto something that would work for Sona in that role. This is what I came up with and what I use.

This IS my first guide, so I do apologize if it's just...horrible. I may edit and tweak it over time.

Down the nitty gritty of the guide...
PROS
[*] High burst.
[*] Amazing mid/end game.
[*] Fun and challenging.
[*] You often get the element of surprise. Very few people expect Sona to come out as an ADC.
[*] Can win most trades and chases come endgame.
[*] Great utility.

Unfortunately, everything has ups AND downs...
CONS
[*] Very, VERY squishy. Sona is the epitome of a glass cannon with her Blade of the Ruined King coming late.
[*] Slow start and can be hard to keep up with a bad support.
[*] Mana hungry early game. You'll have to spend the first few minutes rationing mana until you get your Tear of the Goddess and put a few stacks on it.
[*] Your team will possibly rage. I've had people dodge lobby after lobby because I wasn't following the meta. Offer to prove them wrong.
[*] The absolute worst part? Unless you can kill one of them off with your initial burst, you can NOT solo a group. Sona is group reliant. If you manage to catch a group of 2 or 3 enemies with your stun, you'll probably win that. If you don't have your stun and wander into a group alone, you're probably dead.

I don't really know a whole lot about what runes are absolute best. What I have listed here is a pretty standard spread for phys damage games.

I'm proud of how I managed to squeeze some multipurpose utility into the build. This puts her at 85% (with [statikk Shiv]]) crit with over 300 AD, a near endless pool of mana Muramana, respectable move speed without boots (426 if you choose Zephyr, I THINK), if you use her E to travel (you should be) and a ridiculous burst that can send even the tankiest of tanks stumbling back to hesitate.

You want to max Hymn of Valor first. It's going to be your most important skill and will be how you lead in a lot.

What this guide basically leads up to is a very VERY powerful glass tank ADC with some decent utility. If played well, you'll be dishing out damage that anyone would respect. I often out damage anyone else in the game by double or more (may try to nab a few screenshots of future games). She has a slow start, but good support deals with that with no problem, and she's squishy, but by endgame, very few champions have much time to react to the damage you can deal.

If you manage to pull the game off, you'll be a major asset to your team. You have a heal, that while isn't superb, is at least useful. You have a move speed buff that makes you easily one of the faster champs on the map, an ult that can stun an entire team, and a high damage harass/burst.

You'll have to learn to last hit at the last moment. While Sona's auto missile speed is actually pretty quick, her starting damage isn't high. At all. You'll have to master hitting just before minions die over the first few levels.