Like user harikris suggests, I would highly recommend you move all your audio playback and processing code to Android NDK using the OpenSL ES library for the best performance.
As I understand, the AudioTrack API is built on top of OpenSL ES Buffer Queue Audio Player. So you could probably improve performance by working directly with the NDK, writing C code that is called from your Java/Android layer to work with the sound.
The native-audio example mentioned above contains code that will show you how to play a sound file directly from a URI. In my experience, the results from this method are better than AudioTrack in Static Mode.
Soundpool is generally reserved for very short sounds that can be played from memory and its not a scalable solution for your sequencer especially if you introduce large files.
Here are some links that have helped me out with my applications: -General Information on OpenSL ES for Android: http://mobilepearls.com/labs/native-android-api/opensles/
-An Android audio blog with some great example code: http://audioprograming.wordpress.com
Edit: The old mobile pearl link appears to be down. Here's a working one:http://mobilepearls.com/labs/native-android-api/ndk/docs/opensles/index.html