Wikipedia gives the equation for the doppler effect. Basically it's to change the frequency of the sound based on the speeds of the source and the listener.
The speeds need to be relative between the objects. For the source for example:
ang = angle(source.x,source.y,listener.x,listener.y)
speedsource = source.velocityX*cos(ang) + source.velocityY*sin(ang)
The listener speed is about the same but uses it's speed instead and the ang is from the listener to the source.
The frequency is calculated with:
(speedofsound + listenerspeed)/(speedofsound + sourcespeed)
"speedofsound" is something you can tweak, I used something like 3400 to make the effect more subtle.
Use the frequency to set the playback speed.
Here's an experiment of it. In it I used positional sound and some volume fade off.
https://www.dropbox.com/s/sfs9uqb7vy9bo ... .capx?dl=1