I play a lot of Sona, and one thing to remember, more so than with my Lux games, is to be fluid with your builds.
To start, your runes look solid, and I'd recommend flat HP Quints. Once you have those Doran's Ring isn't needed, but before then it does help with a little early padding. If you're confident you don't need it obviously go with Meki/Ward. MP pots, if you have Clarity and MP runes, really aren't that useful imo.
Then comes your first choice. Tear, or Chalice? I've had people say Chalice is terrible, but I've had success with it. Your skills use a moderate amount of MP, and you have a good base pool anyway. The extra MR helps a little against nukes early, and with the regen and Clarity you're pretty much able to stay out in the lanes all the time. If you don't feel you need the MR at all, Tear for sure.
Boots are the next choice. I go between Mobility, Swiftness, and Mercs depending on the enemy lineup. If they have chasers you're pretty good with Swiftness and can outrun a lot of them, otherwise Mobility is nice for the general map travel.
I almost always build a Mejai's. It's easy to get stacks with Sona, and it covers your early AP needs well. If you're doing even just okay, you'll still have a few stacks on it.
I've never gotten a Soul Shroud, but that's just my personal preference. By the time I could get it I feel most of it's usefulness is gone. I'd much rather get a Spirit Visage.
Ah, Spirit Visage. I love it on Sona. A little HP and MR, CDR, and the self heal buff makes you deceptively difficult to kill unless they have disgusting burst. I've managed to kite off, heal, and finally kill an Ali TF chasing me with only that, Mejai's, and Chalice before, simply because the heal buff kept me alive enough to keep kiting.
My core in a majority of games ends up with those three items, and a ton of wards.
After that I recommend more Aura Items. Sona is Aura crazy already, and getting even more is nice. My favorites are Abyssal Scepter and Aegis, depending on what you need. Abyssal is another item I get often, followed sometimes by a Deathcap if I'm doing really well. I'll some times get it done, and almost never manage the deathcap unless I got lucky with early ganks and am buff. I've also considered grabbing a Frozen Heart, if nobody else is getting one, but never seem to get that far. But Aura items are amazing on her.
Other situation items are: Soul Shroud (Haven't seen a reason to get it over Visage yet), Shurelia's Reverie, (Very situational but possibly useful, especially if you're not getting early assists/farm at all), Banshee's Veil (If the game is getting later and you feel you could use the spell block/MR), Sheen/Lichbane (If you're balling out of control, I guess, you can get this. You proc it often when spamming skills)
But mostly Sona = Aura Items. You can offer so incredibly much to your team with an Aegis and/or Abyssal.