... working on preview for soundfiles, since gstreamer is being used more.
I don't think I've ever had a tooltip on mouseover for any actual icons, except for launchers in the panels. Indeed, if I create a launcher on my desktop, the "description" and "comment" feilds do not display
as a tooltip. However, these do appear as a tooltip when I move the launcher to a panel.
... suggests that preview play is a matter of installing the appropriate cli player and adding an appropriate script to /usr/local/bin (Havn't tested this)
gconf editor has a bit of a discussion associated with it.
... basically, just cause you enable it in gconf don't mean the actual application is present.