The thing with Locket though is that you're saving up 2000 Gold just to make its base form, and upgrading Aegis to Locket will only give you a 200 HP boosts, while the Mag. resist field is the same (20 MGR).
It is stll a good item though and has helped me out in the past thanks to its activate.
Froze Heart...nah I sorta don't like it on Sona personally, would much rather get randuin lol.
Ardent on the one hand is 3 chunks of 800ish Gold to create. Movement speed is nice on Sona, for example, so she can poke with her Q. Now if memory serves me right, both passives from Lich Bane and Ardent stack since it doesn't stack if both items are the same name.
Randuins gives:
+500 Health
+70 Armor
UNIQUE Passive - Cold Steel: When hit by basic attacks, reduces the attacker's Attack Speed by 15%.
UNIQUE Active: Slows the Movement Speed of nearby enemy units by 35% for 2 seconds (+1 second per 200 Armor and +1 second per 200 Magic Resist) (60 second cooldown).
Where as Frozen Heart gives:
+100 Armor
+20% Cooldown Reduction
+400 Mana
UNIQUE Aura: Reduces the Attack Speed of nearby enemies by 15%.
+500 health is very nice but your main job as Sona in teamfights is not to tank. It's to ulti as many people as possible and then fall back to where your ADC is so you can peel/heal/exhaust for them. 20% CDR and 400 mana help you get more ults off and stay topped off on mana. The AS reduction aura is also godsend against most top laners, junglers, and some AD mids that want to murder your carry.
The MS slow active on Randuins is nice but so many champs have dashes, jumps, and high mobility that it doesn't really matter unless you're in a clumped up group of enemies or chasing.
Aegis gives:
+200 Health
+20 Magic Resistance
UNIQUE Aura - Legion: Grants nearby allies +20 Magic Resist and +75% Base Health Regen.
(Unique Auras with the same name don't stack.)
Against
any aoe magic champ (IE: Kat, Liss, Rumble, Kennan, Xerath, Zyra etc etc) decreasing the effectiveness of enemy poke and giving 20 MR to your carries can be the difference between a won teamfight and complete disaster.
Do you know what 20 MR can do for your team?! Kat's ult can only hit 1 person a maximum of 10 times. Let's say she jumps on your carry and gets 5 daggers off. Assuming 200AP on her and rank 2 ultimate (55+25%AP) so 105 damage per dagger. Without aegis she just dealt 525 damage on your carry, with aegis she dealt 425 damage. That's just one spell on one champion. Imagine a prolonged teamfight with your tanks and carries taking magic damage. The difference becomes huge.
Now Ardent Censor
+40 Ability Power
+10% Cooldown Reduction
+100% Base Mana Regen (+round(1*stats[BaseManaRegenRate]*5) Mana per 5 seconds)
UNIQUE Passive: +8% Movement Speed
UNIQUE Passive: Your heals and shields on another unit grant them 25% Attack Speed for 6 seconds.
(This does not include regeneration effects or effects on yourself.)
So this costs 200g more than an aegis and only gives
you stats. Not only that but if you're a half decent ranged support you should have built mikaels first so that 100% base mana regen is overkill. 10% CDR is nice but FH would give you more. 40 ability power is what, a blasting wand's worth of AP okay. So you heal for a bit more and do a small amount of damage with your Q, neat. But chances are that once you are out of lane and into the mid game, if you're in poking distance of the enemy you're also within engage distance of the enemy. Which is a big no no for a squishy support who built and Ardent Censor.
TL;DR Ardent Censor is bad, get Mikael's first then FH or Aegis depending on situation imo.