Ever wanted audio, text, or video to open up in a different application than whatever Debian has slotted for your default? Want to get down to the nuts and bolts and do it manually?
Well then. Take a look at /etc/gnome/defaults.list for what application is used to open up what extension or mime-type, and bring up the /usr/share/applications/ directory for the configuration files that dictate how that application is going to open up.
For instance, I changed .pls and .mp3 from Totem (possibly the last media application installed) over to audacious2-gtkui for better integration with the look and feel of the gnome desktop.