I found this thread by google and I'm having the same issue on Nostalrius. I'm on Windows 10 build 10122 and I believe the issue may be attached to that.
My sound.log shows nothing out of the ordinary, my game isn't muted in-game either:
5/24 14:06:13.300 Sound::Initialize()
5/24 14:06:13.300 fmod.dll version 3.75 loaded
5/24 14:06:13.321 memory system configured: SMem wrappers
5/24 14:06:13.398 Output system: 2 'Direct Sound'
5/24 14:06:13.398 Driver: 0 'Primary Sound Driver' 00000000
5/24 14:06:13.398 Mixer: 0 'MMXP6'
5/24 14:06:13.398 Buffer size: 50ms
5/24 14:06:13.398 Channels: 12 (12 software, 0 2D hardware(dsound), 0 3D hardware)
5/24 14:06:13.398 Output rate: 44100 Hz
5/24 14:06:13.398 Initialization flags: 00000082
5/24 14:06:13.398 Sound::Initialize() complete
WoW isn't even listed on my Volume Mixer: