Hi
I just saw this video It sounds kinda childish but looks like fun to me: http://www.youtube.com/watch?v=b9XAC-BvUyo

Should be possible on the N900 don't you think

How it works:
SoundRacer has a developed a special technology to sense and calculate the car engine rpm and produce the sounds of a powerful Super Sports Car engine. The sound changes as you drive and SoundRacer even increases the speed of the engine sound so that you can drive at a normal speed and still get a very exciting engine sound. SondRacer detects the small signals from the alternator on the 12V system in the car, it does not use ignition pulses. That is why it also works on diesel cars. Manual or automatic transmission makes no difference for SoundRacer, it works on both types but of course the rpm response is different on automatic transmission cars with hydraulic tourqe convertes, the rpm increases as soon as you press tha gas pedal.

A built in FM transmitter transfers the sounds on a selectable frequency to the car stereo system.

It also works as a standard Car FM-transmitter for MP3 players, such as iPod and iPhone, with a line in socket that provides an input for playing music on the car stereo. A cable with 2 x 3,5 mm stereo plugs are supplied in the box. As soon as you plug in the cable in the line in socket on SoundRacer, the engine sound is disconnected. The other end of the cable you just plug into the headphone socket on your music player and then the music is heard in the car speakers.
Source:http://www.soundracer.se

Please create a brainstorm item and read through the links in my signature.

Found this some day at a tuning site and consider it useless...

but it should be some kind of easy to program a tool reading speed from gps data and playback and modulate a soundfile to jack or radiotransmitter