Taunt is autohit and has a 5 target limit. The proc has a 20% chance to fire on any foe hit by taunt. It is entirely possible for it to hit more than one target.
Chilling Embrace is also autohit, but as it is a toggle power the Impeded Swiftness proc will only have a chance to fire every 10 seconds on foes in range.
Taunt auras do not accept taunt IO sets, so no Zinger proc in Chilling Embrace. 
If you put a proc in a power with a to-hit check then the proc activation is also governed by said to-hit check.
For further info on procs, global IOs and the like, refer to: http://wiki.cohtitan.com/wiki/Table_of_Special_IOs
Hope this answers your questions!
...in CoH racing to 50 is like trying to race to the end of your vacation. -Arcanaville
Debt barely slows down levelling these days. It's just a little bar that measures how much Awesome you've generated recently. (If you're not getting debt, you're just not trying to generate Awesome hard enough.) -Kelenar
Thanks Mr E!
"Comics, you're not a Mastermind...you're an Overlord!"
One other thing to keep in mind: Not all procs have the same chance to fire. On melee attacks, the touch of death is only a 15%, not 20% like Mako.
And a few procs don't do anything. Specifically the Absolute Amazement purple set's proc is currently busted.
"Hmm, I guess I'm not as omniscient as I thought" -Gavin Runeblade.
I can be found, outside of paragon city here.
Thank you everyone at Paragon and on Virtue. When the lights go out in November, you'll find me on Razor Bunny.
Yeah .. I have seen the taunt proc hit more than one target. I have even killed an AV with it once. Good times, good times.
Some Taunt auras used to take those, and do still work if you've never respecced that char :-)
My memory's not as sharp as it used to be.
Also, my memory's not as sharp as it used to be.
"The tip of a shoelace is called an aglet, its true purpose is sinister." The Question
You don't know how hard the decision was to respec my brute out of the taunt proc in his Invincibility. The lure of inherent Stamina was just too great, so I finally went for it.
But dang, I miss being able to stand around slowly killing mobs with awesomeness.
Statesmonkey Sez: Lighten up! It's a game, for Lincoln's sake!
Also: Six years of casual play begins to look an awful lot like one year of hardcore play.
I often use Procs like the Perfect Zinger chance for Psy dmg in Taunt. Taunt has a 15' radius and I believe a limit of 10 targets hit. I've been told that the Zinger will only hit 1 target no matter how many are affected by the Taunt. True or false?
Same question for such things as the Impeded Swiftness chance for dmg and Chilling Embrace.
Sorry if this has been covered elsewhere but a search for 'Procs' give me a boatload of information to sift through.
"Comics, you're not a Mastermind...you're an Overlord!"