My final WAV file of my song is quiet

After I finished my song and exported it as a WAV file of my main mix, I shut off my VS100 and went to listen to the final product in iTunes. I played it through my computer speakers and it was pretty quiet as compared to any other song's considerable volume. It is a simple song with only guitars and a vocal track. The guitar is somewhat clear but the vocals come out very quiet. I listened with headphones and volume was not a problem. I don't understand why it is coming out so quiet through my computer and not through headphones. I would like to know what i can do to make the song louder without clipping it. Thanks