Sona is a very squishy champion, and because of that she's [most of the time] targeted 1st by enemy champions so she can't heal her allies or make any friggin' damage, makin' her useless in team fights. For this friggin' reason, I decided to build her as a tanky support mage, and I find it pleasing how I can turn team fights into our side by staying alive during clashes. Spamming Hymn of Valor and Aria of Perseverance is the way to go. Don't flame me. I know AP Sona is good, but it just doesn't suit my playstyle [got used to playing aggressive champions]. I played AP Sona back then and I was annoyed by the fact that I'm always gonna' be the one to die 1st.

This build is not intended to replace and shouldn't be compared to real tanks like Alistar or Shen. But she can if she pleases or on situations where the team lacks a tank. However, you should be aware that Sona doesn't have the CC skills a real tank like Alistar have. This is why she should be carefully played, but not as much as an AP one though where you should always lane at the back to make sure you don't die and waste your Meja's Soul Stealer stack [if you build one]. This also makes her Hymn of Valor harder to use b'coz of its low range or risking your life by jumping to your deadzone.

Pros
-High survivability TANKY
-Very good support
-Low aggression coz of high HP
-Initiates team fights with Flash Crescendo
-Survives clashes most of the time

Cons
-Damage and Heals are not high but acceptable
-Should be played aggressively but VERY carefully

For Items

CORE ITEMS
Catalyst the Protector should be build 1st, the passive is just too cool to pass up and will be useless later. Then this should be RUSHED into
Rod of Ages gives you high HP Mana and AP, and also to take advantage of the bonus it'll give later.
Ninja Tabi for extra survivability.
Rylai's Crystal Scepter for more HP and AP.

Then either
Frozen Heart if enemy champions are mostly auto-attackers or
Abyssal Mask if enemy champions are mostly Magic damage dealers

Luxury Items
Force of Nature, Rabadon's Deathcap, Zhonya's Hourglass
or any other items you fancy. :]

PRIMARY RUNES

9x Greater Mark of Critical Chance 9x Greater Seal of Armor 9x Greater Glyph of Magic Resist
1x Greater Quintessence of Critical Chance 1x Greater Quintessence of Armor 1x Greater Quintessence of Magic Resist

Crit chance marks for early harass and last hit minions.
Armor and Magic Resist seals and glyphs for survivability.
Quintessences compliments your marks seals and glyphs.

Clarity and Flash

Clarity - to make sure you never run out of mana.
Flash - to initiate, escape or chase