From time to time, I need to do this also. I use xmms as my answer. Click on the face of xmms, go to options, preferences, there you can select output plugins. One of these is "diskwriter". "play" an mp3, and there's the .wav in your home dir. Don't forget to put it back to what it was before.
Now, K3b certainly CAN do this automatically, IF you have certain plugins available for it. I do have them, but, I had best leave this to others, as I am not sure what the required plugins are. I might also note, that, no matter what I do, rewritable CD's will NOT play in my G.M. truck, only the one-shots work.