As this does not happen with GNOME and has not been reported for Fedora 12 or older, does anyone know what the problem could be?
[ https://bugzilla.redhat.com/600945#c7 ]
With a fresh install of F13 KDE x86_64 and default KDE settings, Audacious starts with dark-grey fonts, which are hardly readable: https://bugzilla.redhat.com/attachment.cgi?id=421745
System Settings > Appearance > Fonts
Here I don't need to change anything. It is sufficient to click something and revert it again, then click the "Apply" button which gets enabled by that action. Immediately afterwards, the fonts/dialogs in a restarted Audacious become readable until next reboot.