Don't like the choice of boots of mobility and the shroud

Seems like a decent build, a couple suggestions though.

I like to build at least one philsopher's stone on Sona, since as support you should be giving most of your last hits to your laning partner, meaning you'll be short on gold, and the regen makes laning easier.

Also, Expanded Mind works only off your base mana, so at level 18, it nets you an additional 1120*0.05 = 56 mana. With Strength of Spirit, that's only an additional half a point of health per five seconds, so I'd suggest taking the points out of Expanded Mind to max the exp boost and put a point in increased buff duration (if no one on your team needs blue, it can work wonders on Sona especially when you're building a Tear imo).