EDIT: Fixed the behavior by commenting out in /etc/pulse/default.pa the line
load-module module-switch-on-port-available
by adding a # to the front. ALSA was handling the muting on its own anyway.
Man, this is the third thing I've had to alter in default.pa to keep my audio working properly... My favorite was the one that randomly stopped VLC every time I got a Skype message until I commented out the module.
]]>Check if you have anything pulse-related installed. For me having libpulse is okay, but anything else would mess with alsa.
pacman -Q | grep pulse