I think you're going to want something like this.
An op amp circuit to boost your mp3 levels to 0-5V range, and an envelope follower circuit to smooth out the level and not drive your servo totally batty.
I recommend you get some oscilloscope capability so you can capture the output be able to tweak the values.
www.dpscope.com has a very nice little USB scope for not very much money that will work great.
