Vehicles sliding everywhere? This is not a bug I'm familiar with.
But yeah, Music discs would be more Minecraftey than mp3's, but possibly harder to implement. You'd need to have a new UI feature or something that holds the disc in place, and allows you to load it/ unload it with the R-key menu. I just love the idea of getting in a car, starting it up, and the radio turning on, and then me driving around the city blasting "Mall" or "13" at random pedestrians. Or better yet, if helo's idea of custom discs were ever added, blasting cool 1930's and 1940's Fallout-like music, since our server (and the content pack I made) have that era's vibe and all.
Also, not to be rude to Golddolphin, but vehicles? They do have radios, in most countries I think. My car IRL can play CD's, mp3's, Spotify, all sorts of fun stuff. I don't think it's far-fetched to say the idea fits into Flan's mod, lol. Should it be a priority feature? Probably not, but hey, if Prototype and others find it worthwhile to do then I'd be happy to see it implemented someday. I don't see how car stereos would ruin immersion.