From the WoW Forums:
This evening we will be taking down the PTRs to apply a new build with more fixes for voicechat and sound issues.
The servers should go down around 7PM Pacific time and we expect them to be down for 4-6 hours.
Once the servers are back up please be sure to report any issues […]

The Kirin Tor realm is down to resolve performance issues. We anticipate that the realm will only be down for a short duration, and appreciate your patience in that matter. WikidraSenior Game MasterBlizzard Entertainment
Original post by Mike Schramm