I put a lot of thought into buying Philosopher's Stone. The problem was, I kept making concessions to keep it in the build, and I couldn't figure out how to balance the CD reduction properly. For instance, it originally was the first item I wanted to buy to allow it to give me as much gold as possible. I would hold onto it the whole game and turn it into Eleisa's Miracle at the end. But this means Chalice of Harmony would have to wait, which I think is Sona's single most important item and vital to get as soon as possible. Then after that you would need boots, so I kept pushing it back until I decided that there just wasn't room for it and it wasn't necessary. If I wanted to turn it into Shurelya's Battlesong, I would need to do it early to get maximum cool down reduction, thus defeating the purpose of Philosopher's Stone's gold per second. Also, in my experience, Song of Celerity does a good enough job of replicating Shurelya's Battlesong's active, so you don't need to purchase it. Also, buying the items I suggest allow Sona to perform better in the all important early to mid game when compared to buying Philosopher's Stone and Heart of Gold. I love Aegis of the Legion and would hope the tank would purchase it (I purchase this item second after boots as Amumu), since it really is a fantastic tank item as well as a support item, and someone on the team should have it, but I felt it wouldn't provide Sona enough survivability if I were to buy it as my fourth item. If you really wanted to, you could buy Aegis of the Legion after Guardian Angel, though I feel Aegis of the Legion loses some of its effectiveness late game. Items like Abyssal Mask and Soul Shroud are expensive, and I felt like most team members wouldn't take Soul Shroud's cool down reduction into account. I thought about Zeke's Harbinger and Will of the Ancients as well, and would definitely considering putting them into my build in place of Zhonya's Hourglass and Rabadon's Deathcap, depending on my team members. I really tried to take costs into account since it is hard for Sona to make money. But her first four items ( Chalice of Harmony, Ionian Boots of Lucidity, Fiendish Codex, and Guardian Angel) provide the right balance of mana regen, CD reduction, and survivability to allow her to support with her skills and be viable late game at a cheap price. I basically made the decision to focus on improving her skills through AP and CD to provide support, rather than focusing on aura items, which I feel are too expensive for Sona.

Think of it this way: If you alternate between Hymn of Valor and Aria of Perseverance every 4.2 seconds, you can constantly maintain both of the auras provided by both skills, meaning 20AR,20MR,20DMG,20AP for everyone plus an additional 20MR and 20AR for 3 of every 4.2 seconds for yourself and an ally. In comparison, Aegis of the Legion provides 15MR, 12AR, and 8DMG constantly to everyone, along with an additional 12AR and 15MR to yourself. Aegis of the Legion costs 1925g. The items necessary to sustain the auras above, while discounting boots, cost a total of 2135g. I think the argument could be made that focusing on CD and mana regeneration may be a better, cheaper, and more versatile way of achieving similar goals.

Also, look how much Sona's healing rate increases when you reduce CDs and build AP. With no CD reduction and no AP, you will be healing at a total rate of ((18*.6+120)*2/7)= 37.37 health/second. With full CD reduction and (18*.6+30+100+140)*1.3= 365.04 AP you will be healing at a total rate of (120+.25*365.04)*2/4.2= 100.6 health/second. This is a significant increase in healing ability, and shows healing is still viable. Also, remember you heal yourself every cast, which is why I like having Guardian Angel and Zhonya's Hourglass. With those items, I am not afraid to take damage and "act as a tank", thus putting the heals on myself to good use. Plus, the potency of Hymn of Valor and Crescendo practically double with 365.04 AP. All these things make you more than just an aura machine late game.

BTW, I replaced Teleport with Clairvoyance for obvious reasons.

I like this build but I think every support should have the aegis shield. Also have you given thought to getting gold/5 sec items early game? Philosophers stone and the heart of gold are great starting items and evolve into perfect late game items with their actives either drastically reducing enemy dps or increasing your team's movement speed for the necessary gtfo moments. I honestly think your build is great for Sona but I prefer other builds because they utilize her as a team support and utility. The idea of getting starks ferver probably never occured in the making of this build but if you have a strong ad carry, it makes all the difference helping the team as a whole. Thoughts?

Yeah, I thought about doing that. If you started out with 5% cool down, I would most certainly do so to achieve 40% cool down reduction. In this particular build, there is no reason to upgrade Fiendish Codex into Morello's Evil Tome until the very end because the only benefit over Fiendish Codex would be 5 mana regeneration and 45 ability power. The cool down reduction would be rendered useless. Those benefits aren't worth the 1105g to upgrade. My goal was to achieve good mana regen and CD reduction as quickly and cheaply as possible. Morello's Evil Tome would add another 1105g to the current build. It would take too long to get enough money to address survivability.

This build is okay, just that you should probably say that you can turn the fiendish codex into something like Morello's Evil tome.