Before you even attempt to play an exported Mp3 in Media Player, the first thing you should be doing is to export a wav of your song and check it inside sonar, within the same project.
Solo it and route it directly to your Main Outs, bypassing any Master Buss Fx, because you don't want the wav to be processed twice.
Once you're happy with the wav, use that for conversion to Mp3
BTW, this forum is for 8.5 & prior versions of Sonar, there is a separate forum for the X series