Not Updated For Current Season
Alright, so we know Sona is support, that's what she does, so while she doesn't get kills, she assists like a mofo. I'll go by game stage to describe how you make this work. I play 5v5, so this build might not work so great 3v3. I'll also describe 5v5 strategy.
To start with, after buying items, make sure you pick up Hymn of Valor and use it twice while standing on the healy pad and once while running to your lane's turret. Stay by the turret with your partner, and once the minions are behind you cast Clairvoyance in the center of the 2 bushes (do this often). Use the bushes to ambush your opponents mercilessly, using hymn of valor whenever possible to hit the enemy heroes. Be a compete *****, when they're at their turret move just within range of Hymn of Valor, cast, and immediately move away so you don't get hit by the turret. The spell's travel time is slow, use that to your advantage and make hit and runs while they try to defend. On your third cast, make sure to attack them once to get maximum damage. Meki's and the Mana Manipulator make you almost mana efficient unless you're getting pummelled. If you are, focus on healing, defending your teammate and your turret. Chip away at the enemy when possible with her passive.
This stage of the game begins after the first turret falls in your lane, or when the enemy starts ganking heavily. If your turret fell, push back a bit, but if the enemy leaves the lane move to mid to reinforce against a gank. If you pushed their turret, kill remaining minions, move toward enemy turret for 2 seconds so fog of war closes, then b-line towards mid with your partner to gank whoever's there and to push that turret if possible. Spam Song of Celerity while running with your partner to get you there quick. In fights, stay near the center, just far enough away to hit them with Hymn of Valor. If mana permits, don't be afraid to buff your movement speed too during team fights, sometimes it helps considerably. Flip through your other two for sure. Don't initiate with your ult, wait for them to gather against one player and throw Crescendo once 3+ are within range. If your team is losing the fight and running, stay just close enough to your allies to buff them with speed and healing (in that order). Don't leave a team mate if you can help it, but make sure you can get away. ALWAYS stay with a team mate.
This happens once the first inner turret is reached in a fair fight. So if you're getting annihilated or giving an annihilation this would be when the first inhibitor falls. Your job is to keep the team together. Use the boots with your movement buff to race across the map and gather sole teammates, buffing them to get back to the team faster. You will move quicker than most heroes, so make sure to wait when you have to. Once the team is nice and together, you will inevitably fight the entire enemy team, which is where Sona shines. Your items make you have a ridiculous amount of health/armor/magic resistance for a support caster. You're also causing your team to be much better and their team to be much worse, while healing and causing damage. This is a threat that they wont want to overlook, but you'll be able to take such a beating that by the time they get you low you can either flash away, or your team will destroy them. Constantly cast spells, it's not a coincidence that she has a 2 second global cooldown and the Innervating Locket does too. Same rules as Mid Game for how you team fight. Stay with the most wounded teammate if possible, untill they've left the team fight, then get back to it.
So, once again, stay with the team, cast more as the game progresses, and be ready to run if they target you first.
**Side note, I don't think the stats incorporate auras, they should be much higher.