Factor an entity speed is multiplied with for creating a doppler effect with
ent_playsound and ent_playloop.
The higher the factor, the more noticeable the effect. Doppler effects
also occur with static sound sources when the camera is
0 .. 10 (default: 1). At 0 the
doppler effect is switched off.
doppler_factor = 0; // switch off the doppler effect