The simplest way to accomplish this is via jack:

So configure amarok to use the xine engine with the jack-output. When you are using Ubuntu, you need to make some changes in the "xine-lib" package and rebuild it:

diff xine/xine-lib-1.1.15/debian/libxine1-misc-plugins.install 
xine2/xine-lib-1.1.15/debian/libxine1-misc-plugins.install
21c21
< debian/tmp/usr/lib/xine/plugins/*/xineplug_ao_out_jack.so
---
> #debian/tmp/usr/lib/xine/plugins/*/xineplug_ao_out_jack.so
diff xine/xine-lib-1.1.15/debian/rules xine2/xine-lib-1.1.15/debian/rules
57c57
<       --with-jack \
---
>       --without-jack \

Download edcast-jack, do the usual ./configure, make (and probably make install). Adjust the configuration file in conf/sample.cfg to your needs.

Start jackd: jackd -d alsa

(Re-)Start amarok, play something Start edcast: ./edcast -c conf/sample.cfg xine:out0 xine:out1

Now you should be able to listen to your music played with Amarok via your icecast stream. If you want to have a bit more control over the stuff, which is streamed, have a look at "jackeq" which basically does the job of a mixer. To control the wirings inside jack you could use "qjackctl".