Comment 1 for bug 1071213

It appears as though this is a Rhythmbox bug, as playing audio with a different media player (in my case, mplayer) does not keep the screen on.

It appears that the reason that the screen is staying on is that Rhythmbox is sending a InhibitorOn signal to org.gnome.SessionManager over dbus, which keeps the screen on, until the audio is finished playing. This appears in the power-manager plugin, located at /usr/lib/rhythmbox/plugins/power-manager