The Mandolin gets a little quiet in the mix. Don't let your lack of confidence deter you - you're playing it well, and the fact that it's human actually adds a lot MORE to the mix. Your playing is good. Play the crap out of that mandolin and do it every day, improvise.
I think the composition is also maybe a little static, but the idea is good. Improvising on simpler instruments is really a great way to develop a more dynamic sense, even if you're staying within a mode or a scale - it teaches you how to loosen up musically and it makes adding in complex things into improv a lot easier later.
Heh the male choir reminds me of my own voice in its timbre.
Great work!