When you export your mix, you need to make sure you pick the correct sound qualities and export format. Exporting to MP3 will compress your audio. We recommend exporting to WAV file to keep all the dynamics of your recording in place.
Do keep in mind that when you mix your multi-channel mix down, phase cancellation can occur which may contribute to the change in your sound. This resolved by learning how to mix more effectively, reading books, and learning how others do it to get the best sound in your mix.