If you are going to go with sonic blasts, good choice on defender.
As for primary, whatever flavor you want. All three on your short list fit your requirements.
AM in rad gives you a bit of status protection if you like that kind of thing. Rad is a great choice if you are going big game hunting.
Dark is dark, never a bad choice.
Cold is great on teams with minimal investment in team buffing powers and slots wise, the rest of the set helps you as much solo as teamed.
Whatever puts lead in your pencil.
Time manipulation seems to be pretty good, too. The drawback is endurance usage in the low levels. Only temporal selection is ally only for buffs. Time Juncture is the real deal for damage mitigation. Sonic would definitely add well to the mix of time debuffs.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
Haven't played cold defender solo (sg team), but can say from playing solo cold corruptor, that you could have hard time before Sleet (when you get Sleet things become much easier).
Rad and dark defender are much easier to play solo on lower levels, although my dark had bit more problem with endurance (rad have AM). On the other hand dark gets fluffy...
With sonic as secondary I would choose Rad, although sonic have good synergy with everything, cold and dark make me want to have some rain power (fire or ice).
Or if you have slots, you can make all 3. 
"If you want to win you must not lose."
"Easiest way to turn defeat into a victory is to put on the enemy's uniform"
"Better strategic retreat than dishonorable defeat"
- Il Numero Uno (The Number One)
Cold/sonic always seemed fun to me. I've always wanted a dark maisma def or corr and i love my rad/ice defender. They're all good primaries and cold is very fun.
I've played all three. You can't go wrong. But, if you can only try one, or if you are trying 1 first, go with the rad. AM is a difference maker at low to mid levels for an AT that has debuff toggles. At higher levels, AM is even better.
My first defender really sucked. So much so, I nearly swore off using a defender again. My mistake was taking Storm with Archery. However, after 3 years of staying away from using a Defender, I decided to try again. This time, with a lot more experience under my belt, I chose Kinetics and Dual Pistols. I am exceptionally pleased with Cataclysmic Ariel (I am sure a number of players have teamed up with her). At lvl 35 she heals at 400 points a pop and lay down a ton suppression fire on the meat heads attacking the team. She is okay to solo with; however, I have to be careful in which battle I want to fight alone. Anyway, if you want to heal and defend your team, Kinetics/Dual Pistols is an excellent combo. The only drawback is I cannot rez; however, I can buff with speed, defense, and a few other things. 
Current active characters: Dragon Maiden (50+3 Brute SS/WP/PM), Black Widow Maiden (50+1 Night Widow), Catayclasmic Ariel (50 lvl Defender - Kin/DP), Quantumshock (50 lvl Elect/Energy/Energy), American's Defender (38 lvl Tanker - SD/Mace), Spider-Maiden (15 lvl Corruptor - RB/PD) & Siren Shrike (15 lvl Defender - Sonic/Sonic). My entire stable.
I vote for rad/son.
I'm running one now and really having a good time with it.
We don' need no stinkin' signatures!
Rad/Sonic is the classic GM killer.
Traps/Sonic is another GM Killer, and can be a mini-tank.
Time/Sonic would be a third choice, even more tank, a bit less offense.
I've been thinking about making a new defender. I've decided on sonic as the secondary. I have gotten it down to either cold, rad or dark for the primary but could be persuaded for other ideas. Each have their pro's and cons and can't choose between them so I'm asking on here for advice to help me choose. I'm looking for something that would be good in teams and would be able to solo as well. As certain times of the day it can be tricky to find/form a team. Any help would be greatly appreciated