I basically changed the entire guide after playing some ranked games with Sona. I gave up all together on getting kills and I'm not focused on the fact that you want your bottom lane to win. That means harrasing, having no minion kills and feeding whoever is laning with you.
Don't worry about kills, just concentrate on buffing your team and kills will come.
Sona has 550 range, that's ALOT. That's like ashe. If your in range for your auto attacks, your in range for your Q. It's really great for early game harrass, just let your lane partner free farm all day.

For masteries, I go for the standard 9 0 21, I get the reduced death time over mana regen simply cause you faceroll so much in team fights, the mana regen doesn't do anything for you. Although reduced death time is also slightly useless, you shouldn't be dying that much.

For Summoner spells I get clairvoyance only because I think every team needs at one and it helps to avoid ganks provided you remember to use it every 40 seconds.
I get flash, only cause it's a method of escape, but the summoners I would get are Fortify and clairvoyance, this is because with shurleya's + your E your at 516 move speed. So that's a ghost every minute, and if you get caught, then you deserve to die. It will teach you to not get caught ^^

Runes, I get some AP runes, cause I'm basically zero ap most of the game.

Passive: deals 14 damage + 9 x lvl and bonus magic and a bonus effect depending on Sona's last song played.
Hymn of Valor-Stacatto: Deals double power chord damage
Aria of Perseverance-Diminuendo: Debuffs the target to deal 20% less damage for 3 seconds
Song of Celerity-Tempo: Slows the target by 35% for 2 seconds

Hymn of Valor
Damage 50/100/150/200/250 + 0.7 ap ratio
The damage output from this isn't that bad. at full build with about 500 ap you will have 250+350 damage.
It will hit champions if they are in your range but not if there is like a mob of minions surrounding you. Think of it was a skill shot, try to move away from minions when using.
The aura is AMAZING. 20 damage and AP to the entire team??? Make sure to have this aura on when taking down towers.
Aria of Perseverance
Heal for 35/70/105/140/175 + 0.35
for some burst heal it does a decent job, but your not a soraka and your team shouldn't expect you to give them full health either. In early game it heals for about 48hp...not so much, but you should use still try to keep at close to full health. It will heal whoever in it's range is lowest in health. (as a % so a chogath with 5k max hp and is at 500hp will be healed instead of the teemo with 500hp and max 2k). Aura gives 20 AR and MR
Song of Celerity
Burst speed+ movement speed increase aura > ghost
You should be spamming this in team fights too, it's not as useless as you might think, it might help that melee champ get more hits in, you never know. And it will get your passive up faster.
Crescendo
Sometimes I get annoyed at the 1.5second stun, that's hardly anything at all. But it is amazing if you can capture the entire enemy team and your own team accordingly responds.
And teehee it makes then dance ^^

You need gold per 5's. If your lane partner is really passive, I suggest you get another one. I'm quite serious about the zero minion kills. You'll get a lot of money from assists, so don't worry about it.
You want to get your tear of goddess as early as possible but you also want shurleya's fisrt. The two aura items are situational. Soul for more AP team, Aegis for more AD team. you need both at the end though.
As for soulstealer, a lot of the times I forget to buy this item, but it's good for cheap ap. If I don't get it, then end game I'll get a death cap.
I always get merc treads on my supports, but if there is really no cc then sorc or CDR boots are fine. if you get CDR boots, you can switch out soul shroud and get a lichblade

and ward ward ward.
Be a ward *****, I don't care. Have like 6 wards in your inventory, it's not a waste of money.