Controlling volume with sensors

Hello, I am new to using sensors, Max, and Arduino and I'm trying to use sensors to gradually increase/decrease the volume of the audio files in my patch.

The end goal of my project is to have three framed photographs, each with a corresponding audio file and IR sensor. The sensor will play the audio and increase the volume as viewers come closer to the image. My package with the IR sensors has not come in yet so for now I am using three photocell sensors.

I have the input from the sensors going to three audio files that play when triggered by the scale and change (sorry for not describing it well, I don't know all the terms yet). I want to be able to control the volume level with the input from the sensors but I can't find anything about it online.

Any ideas? TIA.