Voice chat has been enabled on the PTR with the patch today(August 10th). For those of you not familiar with the Public Test Realm, it's a server(or set of servers in today's case) where Blizzard allows players to go to help test upcoming patches.
This patch is no different, especially with a new feature as huge as in-game voice chat, so Blizzard will no doubt need everyone's help to hammer out all of the bugs with the system so that 2.2 can go live in a timely manner.
With that in mind, we here at Curse encourage everyone to help out on the PTR by testing this new feature.
To get started and access the PTR, you'll first need to visit the PTR Client Download page. Once that's done, don't forget to copy over your characters for the testing phase by visiting the Character Copy page. For 2.2 testing, Blizzard has enabled both your own character copies, as well as pre-made characters using templates created by Blizzard.
Nethaera has also posted an extremely useful Voice Chat FAQ on the PTR forum, so head over and take a look.