When you play Sona AP everyone will underestimate you. Everyone things you are a troller and you have to earn their respect to start a game and be able to play mid. Which is not the most easiest thing, because for most people Sona is a support. And sure she is!. But AP Mid on <1250 ELO. She is the most easiest champion to dominate your lane completely.
When playing Sona you cant be worthfull late game if you dont win your lane, if you lose lane as Sona (which is less "good" for instance than Brand late game) you cant be the pro carry you want to be.

Normal caster runes:
- Magic penetration marks, extra damage.
- Mana regen seals, mana makes you able to spawn your abilities which will lead to lane domination
- MR/level Glyphs, more magic resist => longer live => more ability spam.
- Ability Power Quintessences, 15 AP -> 13 more damage with Q. Small but important difference in lane.

9-0-21
- 9: is especially for the 15% magic penetration, which is alot!
- 0: defense is nice, but especially the 21th point of defense is great. But we dont have that much points.
- 21: Less cooldown on abilities and spells, the last one is very powerfull because when you make a kill with flash and ignite. At your next try you got your flash back and your opponent doesn't so he cant escape. Also the faster XP ganing is important to overlevel your opponent.

If you die alot playing Sona AP you should change masteries to: 9-21-0