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.