Here is a simple example how to write your own synthesizer in Astro with multiple voices and a simple ADSR envelope. I thought I write it so that not everybody is posting hurtful high-frequency oscillators.
You will see why latency really matters when programming audio applications once you start pressing some keys. It is not really possible to play anything but hopefully this well get adressed before the final version of the FP10.
Also there is currently not a really great way to develop with FDT for Astro. It is too bad but I hope the guys at Powerflasher will release an update pretty soon as well. Right now you can compile the example using Ant.