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 you encounter with sound or voicechat here in this forum or via the in game feedback tool.

UPDATE (7:45PDT): Looks like we were a little delayed, the servers are going down now. Still estimate 4-6 hours of downtime.