Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Magic Pen reds
Mana regen per level yellows
CD reduction per level blue
Mana regen is nice since it allows for more song spamming and you wont be spamming as much early levels so the per level ones are better.
CD reduction will allow for more spamming at higher levels.
Magic Pen and Health quints are just good overall.
Get ap and magic pen in offense.
Exp gain, mana regen, move spd, cd reduction in utilities, also making sure to get the upgrades to flash and clarity. You will not need golem buff with this build, but sometimes you will end up getting it so we get a point in utility mastery.
You may consider clairvoyance if no one else on your team is willing to get it. Clairvoyance is very important, but clarity fits Sona better since it allows for prolonged fights and pushes, which is Sona's specialty.
Regardless of what some other people say, Sona is not that great of a babysitter early game. The heal starts off only healing 40+5 on a 6 sec cd or so and does not scale with skill level particularly well. You should instead focus on hymm of valor, which scales much better early game, and play aggressively, last-hitting and denying the other team exp and gold. Song of celerity is not particularly useful while laning so I put that off until 8 in favor of an extra point in heals.
Start with Sapphire Crystal and 2 health pots. On return, build it into a Catalyst, one of the best early game items because it essentially gives you infinite mana until level 10 or so and the heals have saved me on more than one occasion. You can rush Mejai's if you are doing well, otherwise get boots first. We aren't in a rush to build Locket since we want to milk Catalyst for all its worth, so we build Spirit Visage first, provide much survivability and CD reduction. Visage combos well with Locket and your heal since it always heals yourself. Then get Locket, at which point you have the core of the build, which should give you nearly maxed CD reduction. At this point, you should go for Zhonyas for more damage, bigger heals, and anti-ff active. Last is Guardian Angel with will keep you and your auras around even if you get focused and helps you keep your stacks on mejais.
The problem I find with this is that excess mana gives diminishing returns and with clarity, mana should not be a problem. Zhonyas gives more AP and survivability. Also Sona's AP ratios aren't the greatest not because they are low, but because they dont synergize well. For chars like Annie and Morg, stacking AP is good because all their AP goes into damage. The more burst damage you have, the better stacking AP gets since it makes you all the more likely to burst someone down. But Sona has some AP going into damage and some AP going into heals so adding AP does not increase her burst ability any. A bigger heal is nice, but its not worth an 2200 gold and an item slot.
The stats it gives are mediocre and is really only useful for its speed buff and CDR. But since we already get max CDR, this item becomes unnecessary since Locket and Visage gives more survivability. Stacking movespd loses to any slows, stuns, or snares.
Just because Sona is already a aura bot does not mean you should stick more on her. She needs survivability and CDR to make a difference. With that said, Frozen Heart is a decent choice if they are heavy melee. Soul Shroud is justifiable if your team is very cooldown dependent, but gives much less survivability then the other core items.