In the 2004 #7 (the AM package) you get the single play CD and non-JBL stereo. No amps, no extra speakers, etc. Just a CD player. The top-o-the-line JBL stereo (BC package) is a changer/cassette combo unit.
I had the six-disc CD changer added to the pocket below the standard CD player, but I have not heard of a cassette option. Dave at Coastaletech.com has an audio/video input device forthcoming that would allow MP3 players (as well as DVD players) to play thru the Prius stereo (regardless of standard or JBL stereo). Check out that site. I would think a portable walkman-type cassette could be plugged into this input as well. Hope this helps. Love my Prius!
do be careful to note your model year - all 2001-2003 Prius come standard with a cassette player, which may confuse some readers... (obviously you have a 2004 or newer Prius, and no, I haven't heard of a cassette player addition yet.)