Converting Midi to Audio

If I set up my juno di with 16 midi tracks and Cubasis records my tracks as midi and plays back perfectly, then I should be able to convert the midi to audio so I can render with effects. At this point when I mix down it makes this fast musical noise and then the mix down is silent when i touch the playback button. Weird. When I mix down midi from the sounds in Cubasis (terrible sounds) they do mix down correctly. Why?? If I have to record from my Juno as audio then midi is useless to me. The Juno is not a work station.