From TM Wiki

Since TrackMania Original/Sunrise Extreme. People can change the way the engine sound of their car. This is done by adding the following files to the car's zip file that also includes paint and 3d model data.

• EngineRev2.wav (or ogg)

Faster reverse gear taxi-ing

• EngineRev.wav (or .ogg)

Slow reverse gear taxi-ing

• EngineIdle.wav (or .ogg)

The Stationary rumble sound of your engine

• EngineLow.wav (or .ogg)

The base component when taxi-ing your car at lower than 50 kph

• EngineMid.wav (or .ogg)

This component of the engine sound will be present in second and third gear.(50-180) and will be slowly faded out in favor of the next component when you cruice faster.

• EngineFast.wav (or .ogg)

This component of the engine sound will be slowly mixed in when you are a gear away from driving in full speed.

Some internal technical rules apply for all engine sounds:

• EngineRev's doesn't aply in the TrackMania: Original environments: Snow, Rally and Speed

• If you want to hear the effect of driving your car in the wet sectors of the environments Stadium, Island, Bay and Coast: you need to create stareo samples of your engine and use software like GoldWave or SoundForge to apply a chorus and a cutoff filter to the right channel of the sound file. The game will then use this channel when you are underwater.

Tips

• It's preffered to use .ogg files to make your car's zip file as small as possible

• Try to minimize each sample length to one second or if possible 500 miliseconds.