So this is what I think you are doing. You opened the midi file that was created on another DAW into Sonar as midi tracks, and then insert soft synths to play the tracks?? correct?
But somehow you are saying you "converted" the midi to audio?? this part is not clear.
Did you bounce each track to audio? This is not nessassary in Sonar as you can just export the midi song and it will create a Stereo wave file of the song exactly as it sounded when played.
If the original song used GM midi then TTS-1, as example, will play the song more or less the way it was composed. You simply OPEN the file in Sonar, insert TTS-1 and set all track outputs to play it. If there is peaking then you need to turn some tracks down.
Bouncing was used more in the past when we wished to reduce CPU load. Not really needed anymore unless your going crazy with 25 soft synths.