Does anybody know of a list of D-Bus events it would be useful to subscribe to? I am thinking of events that say "network is down/up", "sound has been muted", ...

There probably isn't a D-Bus event for "sound has been muted", but I think there should be. Audio/video players and (some) games should pause themselves when they receive that event. I don't think they should unpause themselves automatically when the sound gets unmuted, though.