Mp3 is a lossy format, so each time you save it you lose quality. It's going to sound like the plague after all that. Then import it to your project, then mix it and save it as an mp3, yet again. Then you record it though your soundcard, so it's being converted from digital to analogue, then back to digital (and if you have a cheap soundcard, the less you do that the better). Then Youtube re-saves it, compressing it again. It would already be compressed and most likely in mp3 format when uploaded to Youtube. The trouble is that the audio will have been re encoded as an mp3 many times, by the time you mix it.
