You're build is super squishy o_O Even though you have a spammable heal, you have no defenses to protect you from any incoming damage early game. I'd go with Greater Seal of Health/ Greater Seal of Scaling Health for yellows, since Armor seals were nerfed. In addition, I'd highly, highly recommend you get some MR glyphs as well. They'll reduce the amount of damage received if you're going against an enemy AP carry, which subsequently reduces the amount of heals that you'll need to use.

I don't think it's possible to max your Q by level 8, nor is it possible to add yet another point into a fully maxed ability. I know your E isn't very useful as it scales, but come on...

No Spell Weaving ? It works extremely well with Sona's passive.

I find it ironic that you've placed such a huge emphasis on how Sona has early game mana problems, but you didn't mention getting Chalice of Harmony. This would help with mana hungriness exponentially, and also gives some much needed MR (considering you don't have any except your base Armor/MR because of your rune choices...) Plus, later you can upgrade it to Athene's Unholy Grail (which imo, is a really good item on AP Sona) for the 20% CDR. This way, you can cycle through your abilities more, and proc more Power Chords.

Void Staff is not situational! If you are the APC of your team, you need the Magic Pen! How do you plan on doing any damage late game if you can't get through the enemies' MR?! I'd replace Deathfire Grasp with Void Staff. Enchantment: Furor is an interesting choice. I never actually though about it. I suppose it might work, considering how Sona relies on her auto attacks more than most APCs. I'll have to try that sometime.