IMovie Ruins Audio Quality.....Help!?

hey, i just bought a Apple Macbook Air the other day and i started using imovie so i imported my pictures and etc. then imported my MP3 song in the background. When I rendered the video and it was done, I played it and the audio quality didn't sound as good as the MP3 itself. does iMovie ruin quality of songs? i double clicked the song and turned up the volume a little bit, then rendered it again but didn't do much....what should I do? i wan't the audio quality in the video to be the same as the MP3 itself....any steps?