So the idea behind this build is to keep Sona alive to do her annoying poking, healing/shielding allies and running. As I main Sona my basic problem is instant death. Warwick ult with a Meta build Sona for example usually means you die. If you forget to look at the map because you´re too busy trying to help your carry kill on bot lane you might miss an enemy jungler coming your way and you die again, it really sucks. So get some defense, but you still need CDR. All these items give CDR, 2 of them give 20% CDR. So the CDR remains the same as when I normally play Sona. I can cast spells just as quick as I normally would, I just heal a little less and poke a little less hard until I get my power chord. That´s where the AD runes come in.
Sona´s passive Power Chord give a special effect to Sona´s next base attack based on last spell cast. I rarely use any other then blue Power Chord because the DMG is awesome, AD runes just let it do even more DMG (sry, ks.. my bad!)
Ofcourse Sona will never be as tanky as Malphite or any other tanky champs but this build may help you survive the enemy focus long enough to throw your ult and take a safe position behind your team where you can buff, heal and poke from a safe distance.

Updates will follow ASAP. In the meantime let me know what you guys think of this build.