Audio Volumetric Audio Listener? Scale Audio Listener?

I have a scene setup with an orthographic camera looking directly at a plane with an image on, the camera can move across this image and also zoom in/out.

I've placed a number of audio sources set to play on loop around the image with 3d falloff settings.

I'd like the audio to be fade up when the audiosource comes into view of the camera, however as I understand it the audiolistener is a point so audio is only full volume when the audiosource is directly in the centre of the camera view.

I've played around with falloff settings to try and achieve what I'm after and I think if the camera was just at one zoom level this would be enough but as the camera zooms in and out I'd like the sources to raise and lower volume based on them coming into view at the edges of the camera.

It'd be great to know what people think the best way of achieving the above behaviour would be.. I rarely use audio in such depth so perhaps there's a trick I'm missing.