keyboard connected to line-in jack...but only plays out of one computer speaker?

[hey]

I have my audio cable connected from my triton l/r plugin ...
to the blue coloured computer line in connection....

volume recording properties has line in checked.
no volume is turned down. everything looks ok....

but when I play the keyboard...it only comes out of one speaker...?
and then when I play mp3's off the computer...the sound
comes out of both speakers?

I'm real lossed...dont know what to do....
it only comes out of both speakers....
when I plug cable into the mic-pink coloured connection...
but I have read that it's not good to connect
audio equipment other then mic's into that slot...


if someone could help. would be great. thanx!