I connected my RPi onto my stero, so I can listen to my music but I want to listen to a few internet-radio-streams. The streams are available in ASX, MP3 and some of theme through SHOUTCAST I've already installed MPD and ICECAST and they are working correctly. How can I listen to these audio streams ?
You need a client that can connect to MPD and tell it what to play.
The simplest is the command line client mpc that can run on the Raspberry Pi. After installing it, you would do something like:
The URL to add will of course be different - it will be the address you set up in Icecast for your stream.
There are many other MPD clients available, for example MPDroid that runs on Android phones, and can connect over a local Wifi network.