The speaker output is a low impedance (sort of like 'resistance' but more complicated!) - the line in jack is at a medium to high impedance (the mic input is at a much higher impedence, and this can give troubles). Connecting this way you will be OK, usually - just watch the volume output from the player - if too low you will not have enough signal relative to the noise, and if set too high if will distort. Test it - Crank it up slowly till it is distorting, then back it off a little on the loudest portions of the music. This level should then be OK for the whole recording - I could get far more technical, but ... ;-)

You might need to fiddle with any bass/treble controls to get the best sound BTW too... make sure though that when monitoring it thru the PC spakers, that they are not the real cheapies - they may not have a good bass response. Good quality headphones may assist when doing the setup testing.