I'm uncertain how you came across me. However i will try to review this the best i can. I also answered your msg but was having problems sending it, hopefully it went through.
It's not bad for using the limitations of garage band. But i can't let that waiver my review as a musical piece in total.
Things that need some work:Off the bat i can say it's way too silent, but you already knew that. Sadly i'm uncertain the problem causing it using garage band.
Your mix was muddy, meaning you let all your sounds converge into one channel and play through out the song with out mixing or mastering. That leads me to mixing and mastering, here is a site that was recommended to me that helped me with eqing a bit. EQ-ing http://www.tranceproduction.com/techn ique/equaliza tion2.html
Your transitions often times didn't lead into the next section well, it more or less droped the entire song off and went to the bass for extended periods of time. That is fine when using a bass lead, but you need some ambiance or added sounds to help in establishing a song rather than 2 min bass solo. Some pads or low attack sin/saw sounds can help with that. Even square wave arpeggios could aid in establishing a key or mode to the bass line.
hope that helps in some way
Its really hard to mix and master songs in garage band. thats why I have been looking for other tools.