Oh I see. I read speculation somewhere (now of course I can't find it) that the "Speaker Type" might affect how DirectSound3D sound gets processed. I suppose I could run some tests using dxdiag for different speaker types for the same speaker configuration. I would only be using my ears (not very good instruments!)
You do not have to reboot after changing the settings -- whether you write directly to the registry, or use the managed DirectX interface. I tested this by listening to a DVD-audio disc with 5.1 sound using PowerDVD while changing the speaker configuration. When I changed the setting (again, either method), the sound cut out for about 2 seconds, then came back on. The changes were definitely working without a reboot.
I ran a similar test using iTunes, with the same results (i.e. sound cut out for about 2 seconds, and came back with the expected settings).
Wow, WMP11 and Managed AudioVideoPlayback don't