Cold/Sonic is indeed a powerhouse. Going for recharge, as you did, is very important, as you want to get back the better debuffs as much as possible.
Screech is generally a better pick than Shout for a teaming attack chain. Even though Shout has the better damage, its long animation hurts your debuff stacking as well as preventing you from reacting for longer if something untoward happens during its animation. Shout is still a functional and useful power, but Screech is important as well (and it is also a useful control, very helpful when soloing).
Sleet is not an attack and is better slotted with common IOs, or a mix of some sets, procs, and recharge. With your levels of global recharge, your slotting is not a problem if you really want those bonuses, but I really like having a proc or two in Sleet.
I'd rather slot Summon Disruptor with Expedient Reinforcements instead of the purple set. Not a big deal, but when I focus on recharge I tend to really focus. 
When I built mine, I had a different focus. I was going for cheap and well rounded. I went for +hps and tried to get a variety of defenses built (focusing on Sm/Le/Ran/AoE). I had to sacrifice recharge though, which helped me keep it cheap.
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Why Blasters? Empathy Sucks.
So, you want to be Mental?
What the hell? Let's buff defenders.
Tactics are for those who do not have a big enough hammer. Wisdom is knowing how big your hammer is.
Purple control sets are not too expensive. I went all-out for recharge, and could only fit a little more if I could afford purple damage sets. Perma-Benumb, Heat Loss, and Disruptor pet are very nice to have.
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So as per the title I am seeking advice on a cold/sonic build but I also would like to confirm my assumptions on cold/sonic.
After a long journey, I have decided to commit to a main: Cold/Sonic Defender. With the coming "End Game" I wanted a character that will add significantly to any TF style team he find's himself in, with at least a modicum solo ability.
The following are my assumptions on cold/sonic as I haven't played the combination yet, I invite any to correct/comment or add to anything of the follow assumptions:
On paper at least, cold/sonic looks to give tremendous debuffs, with the possibility of stacking 90% resistance debuff, with only /sonic or /TA giving more possible -resist but at the cost of -regen. It also has unique debuffs such as benumb that can be used to good effect against the arch-villain and above level encounters likely in end game content.
Cold also has excellent buffs, which should nearly erase any team members endurance problems, and give something 30% defense to all and 20-30% resists with ice shields and arctic fog.
The combination of strong damage resist debuffs, good single target damage from sonic and perhaps some shenanigans with siren's song cold/sonic should be able to pass through most solo'able content with relative ease.
My very rough draft build:
I took the medicine tree over finishing off leadership, medicine just seems more useful to me being that cold has no other heals, I could be talked out of this but it made sense to me. Also I don't know much about sonic or cold so my general approach to power selection and slotting might be a bit off, advice and criticism welcome.
The build might be somewhat expensive, but to me it is on the extreme lower end of the IO build expense spectrum for me, so I'm open to any ideas as far as "pimping it out". I would say "unlimited budget" but I've afraid of a build just trying to max inf cost rather than effectiveness, like the purple filled 210% recharge perma-PA builds, except on powersets that have no conceivable need for that recharge and would benefit more from increased defenses..
Also,
Many would probably consider this a more personal topic but I would also invite ideas for character story development(power source,mechanics/physics behind a sonic/cold powerset, implications of wielding cold/sonic in the fight for justice, likely nemesis, etc)
I thank all who take the time to respond with constructive criticism and advice and I will be reporting back with my character's progress as I progress with this character.