How to create a Doppler effect with Vuforia Unity3D?

I am implementing an AR application in Android with Vuforia and Unity3D. I have added an audio source to my image target and I would like the volume of the sound to either increase or decrease depending on the distance between the phone and the image target. Basically, I want to create a Doppler effect. Does anyone know how I could achieve that?

I have tried playing with the Volume Rolloff as well as changing the Max Distance in the 3D sound settings, but nothing happens with the sound.

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