You can tune the sound in runtime via "snd_tune".
So you can achieve seamless pitch / volume blending.

Pseudocode:

var vSoundhandle;
var vVolume;
var vFreq;
vSoundhandle = snd_loop(sndEngine, 100, 0);
while(vEngineRunning)
{
vVolume = vCarSpeed*vVolumeFactor;
vFreq = vCarSpeed*vFreqFactor;
snd_tune(vSoundhandle, vVolume, vFreq, 0);
wait(1);
}