How viable would a sonic/storm corr be?

There's no incredible synergy to speak of, but it would be a solid character. Sonic being fairly end-efficient helps a lot with Storm being fairly end-heavy.

Suggestions: For survivability, build for softcapped (45%) smashing/lethal defense. With Scorpion Shield, a Steadfast +3% def, Weave and Steamy Mist you need very little in the way of bonuses to get you there. It'll greatly increase your survivability, which helps a lot with the aggro you can draw with /Storm.

After that, build for recharge and recovery. Get lots of end rdx in your storm powers, the Sonic blasts will generally be fine on endurance with whatever they get from a set.

I've got a guide for Storm in my sig if you want to take a look. I personally went with Ice Blast, but you may still find it useful for general Storm stuff.

For PvP...eh, I wouldn't get your hopes up. I know Stormies were pretty good disruption in Arena way back, I dunno about now. Workable, I guess? Not very good though.

I'm far from knowledgeable about PvP but I see the same suggestions for almost all PvP builds. Build for as much +hp as you can (ideally capped or close to), 41+ points of KB prot, proc out your attacks, get Phase/Hibernate, get SS and SJ.

The problem is, unless you dual-build you're either going to have a crap build in PvP or PvE, because aside from the +hp most of the stuff you really need in PvP is largely a waste in PvE and vice versa.

Hope that helps