Really good guide overall, good item build.
I do however feel like being that far over the CDR cap is a waste.

If you swap the boots our for merc treads you will barley notice the difference in CDR. You gain the MR and the 35% tenacity which will allow you to not rely on cleanse to get out of CC.
You can swap out cleanse for either imp clarity or clairvoyance. Clarity is my choice since it allows me and my partner to harass very hard in early lane to the point where you can deny exp. It also frees up my build to not have to buy nearly as much mana regen. You could skip your tear and just build morellos.
The extra gold would let you build your sheen much sooner which makes a really big impact in lane.

Just my thoughts

If your interested I have a guide that is about the tricks and mechanics of sona you can check out.

ok first off, at lvl 18 your power chord will do 176 damage. That is exluding all resists and such. Now if you have Valor on it doubles to 352. Again thats without resists, and thats with Valor on, which you shouldnt be doing valor Power Chord all the time, thats just bad support.

LichBane will cause you to be able to believe it or not survive more and kill more. If you prefer mejai's for more AP go for it, but for it to be better then just Lichbane's AP you would need 9 stacks. which in this guide without lichbane converts to 9 assists, and no deaths. If you are indeed in a really good match, it could very well be better, but lets be realistic, not all matches go that way. If you do get to 20 stacks, its 15% CDR is waisted because this guide already covers the 40% CDR max you can get.

Now that was just a AP analysis, on other factors, Lichbane gives you mana, move speed, and MR, all good things to help you survive. more mana means more spells to cast, which can mean life or death in some cases.

my apologies on the long answer, but i believe that should do the justice.

What the....LICH BANE?I think Power Chord is giving enought bonus basic attack damage!

An interesting point, However Banshee's against most magic heavy teams will negate the first spell thrown at you, which quite a few of the common AP champs require combo's. However Abyssal scepter kinda flew right over my head, and i will see about putting it in, because it wouldn't be that bad of a support item (considering you will also have magic on your team). However to have that item over banshee's i would consider risky, but also possible (beware of Ashe's arrow).

However i thank you, for the fact that i realized that i didn't cover the alternatives very well, I will also be searching for other healthy alternatives.

nice guide, liked the focus on cdr, maybe add abyssal scepter as an option instead of banshees against magic heavy teams?