you can play mp3 on your ps2, there is a program called Paradox MP3 player for ps2 - allows you to create mp3 discsthat will play on the black slab.
Having finished my Ps2 install at the weekend I have made the following observations:
firstly to me it doesn't make a good in-car DVD player, especially if like me you have a pre-built in IR Ps2. Main problem is if you boot mount you cannot turn the PS2 on / off without going to the boot, which is obviously not ideal. I thought i'd overcome this problem by getting a DVD remote for the Ps2, I extended the controller port to the front of the car and plugged in the ir receiver - all worked fine except for the on/off switch!! It seems that remote on off will only work with the new Ps2's (in this case buy the DVD remote and an IR extender (bigcattech do one) and all will be cool). There is a way around this if you have th older model, bigcattech.com do a remote power/reset button which you can install in the cabin - its expensive ?50 and does mean modifying your Ps2
second and most annoying thing with PS2 as a dvd player in a car is the lack of resume function. Everytime the ps2 is set to standby, you are back to the beginning of whatever you were watching.
those things said - it does do a good job in resisting vibrations - I have not seen it skip once, it also easier to mount than you would think. I have it in the vertical position to the right of the boot, a couple of strips of velcro on the side hold it against the boot very securely, and maybe the most important thing - it looks very impressive having a ps2 nestling in your boot.
I'm gonna keep in the car until I get a dedicated DVD player, then it will just appear in there for special occassions! I've still got the Psone in there so gaming potential remains.
so basically if your main reason for incar ps2 is films - get a standalone DVD player instead, you get resume, no need for power inverter, SVCD VCD and MP3 playback as standard and no turn on / off problems when boot mounting.