In a car? Don't be an idiot. Play music at a normal volume (so that you don't get pulled over for a noise violation, and so that you can hear sirens and trains and things).
I love reading a thread only to find out it's practically prehistoric! Anyway, I'll start rambling as is my wont.
Two things to say -
1) The good buds at
Central European Union Nanny State Thought Police HQ have made it that ipods and other personal MP3 players are volume restricted now, and I don't think you can flash the firmware anymore to bypass this. What rot. (Hey, flash the firmware - did I sound like I knew what I was talking about there?

)
2) My brilliant invention idea which came to me in a firmware flash of inspiration when I was watching cars recently quite oblivious to the blue flashing lights and wailing sirens behind them - obviously in a little world of their own, possibly with their music BLARING loud and then I hit on this idea, which of course has flaws and fatal arguments against it, but then so does the wheel, I'm sure, and look how useful that's been.
Ok then,
Kennys Brilliant Idea 2009®The problem is how to pasively infiltrate a vehicle from outside if you are an emergency vehicle and the idiots in front of you can't hear your ear splitting sirens or your impossible-to-miss-yet-somehow-missed flashing lights...and if the technology was available to send in an audio message into the car using some kind of sonic beam like "Dweebs, get out the frigging way we're on our way to put out a fire" the driver would probably freak out, panic, and crash. I'm sure I would if I heard disembodied voices in the passenger seat next to me. But what, I thought, would work?
(Cue dum
dum Dummmm!Well, you know on those car stereos these days you can set it to automatcally pick up traffic bulletins off whatever radio station in the area is broadcasting traffic news at the time? You know, you're driving along happily and obliviously with a CD on or the radio blaring away - "Missed Me! Missed Me! plinky plinky piano bwham!" then all of a sudden that cuts out and you get, "and now for the local traffic news, there's serious tailbacks on Hang Mans Corner where it's been reported AFP has been spotted alive and well, and drivers are advised to find an alternate route around the corner of Vigliones Freeway where there's been reports of a naked bowler hatted man running whilst shouting "I never wanted to do the bloody Onion thing anyway" and then it cuts back to music? Yeah - you know that? Well!! Here's the genius bit!! You fit all emergency vehicles with a low range transmitter which uses the same technology as that radio traffic thing and it's constantly transmitting, "Emergency vehicle approaching -please let pass" and bingo!! There you have it! A low cost device which doesn't scare the living daylights out of drivers as well as sirens and lights.
Cons:
(i) You have to have your stereo set to pick up traffic reports in the first place.
(ii) I don't know what would happen if you were already listening to a traffic report at the same time - maybe Lentower can work out whether one signal can cancel out another if marginally stronger or some other trickery was involved like bouncing the signal off Uranus.
I know, I know. Bloody genius, aren't I?