Vanilla Quake can't truly do this, produce a sound that smoothly changes location relative to the listener. Anything done in vanilla quake will have been a hack involving repeating a sound. If you were really skilful at the hack, you could create a sound clip that fades out and overlap multiple copies of it to disguise the trick a bit. But it would never be seamless, and a rocket is kind of the worst case because it moves so fast.

The only way to do this properly would be to create a modified engine that included this feature.