The way nautilus call the program depends of the Exec= from the app .desktop, the audacious one doesn't declare taking any filename as argument, that's probably an audacious bug, adding %U make it work correctly
The way nautilus call the program depends of the Exec= from the app .desktop, the audacious one doesn't declare taking any filename as argument, that's probably an audacious bug, adding %U make it work correctly