"audio decoder to install", what is the file ext is it an .exe that has autorun.inf file !! Not sure cause couldn't understand from this quoted line..... generally ubuntu supports .deb / tar ball files and install by apt-get or synaptic package manager. Not sure, but u amy try this:
https://wiki.ubuntu.com/autorun
OR type as: nautilus-file-management-properties
in terminal and uncheck the checkbox which is as:
"Never prompt or start programs on media insertion" in Media tab. If u use any cd / dvd for the same purpose.