The way that I interpret what you are trying to do, you would not need a
media player at all. The device is the player. The line in takes it to the
sound card via a patch cord and the sound card just acts like an amplifier.

You would use a media player if you connect by *usb*. Not all mp3 players
are the same. Some just play the files and the device is mounted like a usb
drive. You can use any media player such as VLC with these devices. Others
are like the iPod and require an equivalent to iTunes that create a database
and playlist. In this case you would have to use a media player that
supports this type of device. Rhythmbox and Amarok support this type of
device.

I may be
wrong, but you may also have to open up the default sound recorder to
hear music on your speakers from the line in... I seem to remember
having to do that, but sound has always been a little screwy on my
machine. A (likely) more complicated way to listen to music on the
line in is to install Audacity and JACK. They are both in the
repositories. The only benefit here is that you get some extra sound
tweaking options to make it sound better, especially if you have some
great speakers on your box. The down side is (to me) JACK is
ridiculously complicated, and I can only seem to get it to do what I
want by trial and error. When I go to record something these days, I
just start up Audacity, and pray that whatever setting I had on JACK
server previously are still going to be intact and working when I
start it. I don't know why it seems like such a crap shoot...

That's just the thing. Plugging in the audio to the line in produces nothing
in the speakers. That is why I thought it may need some application to
route it through the sound card to the speakers.
I wonder if I have the correct driver in the kernel, is there a way to
change
that in Ubuntu?