if you have an internal card the sounds are generated on the soundcard,so go to the windows mixer get into the record options and select the midi icon to record.... go into cakewalk and set up 2 audio tracks for recording 1 for left 1 for right click record
all being well you should have now recorded your midi song into cakewalk.... now its in audio format you can further mix/add effects to your hearts content... once your happy select the 2 audio tracks and choose export audio you will now have a huge .wav file...
now you will need an mp3 program. i use audioactive mp3 studio.... load in your huge wav and save it out as an mp3