Windows Vista has the nice new feature of sound cards being removed whenever it detects that speaker or microphone isn't connected. That means that Pulseaudio will fail to open the sound card, and the user will have no sound even if the speakers are plugged back in.
I'm not sure what they were smoking when they came up with this, or how to solve it. We might have to keep polling the connection in case something shows up. The newer suspend features of pulseaudio might help a bit.
Unplugging the speakers or microphone during an active session also kills the sound until you log in again.
Same problem with Windows 7.
Another fun behaviour because of this is that you get no sound unless you have both speakers and a microphone plugged in. Just speakers is insufficient.
Same problem with Windows 8.
Note that the problem got mitigated in bug 3222 where we load the waveout module twice, once for playback and once for recording. This avoids the issue of not having a microphone plugged in. We still cannot handle hotplug properly though.