The problem is... When the radio gets turned-up it creates more "noise" and then it gets turned-up some more until it hits maximum.
If you can get-around that,* you can get a microphone board ([u[/u]](SparkFun Electret Microphone Breakout - BOB-12758 - SparkFun Electronics) to pick-up the noise and if you search you'll find more.
[u]Here[/u] is a recent post about measuring sound/noise level.
You can use a digital pot [u[/u]](https://www.jameco.com/z/MCP41010-I-P-Microchip-Technology-Digital-Potentiometer-256-Position-10k-Ohm-Single-8-Pin-Plastic-DIP-Tube_241744.html) as a volume control (or 2 for stereo or a dual digital pot). There are a few different kinds of digital pots (the way they are controlled) and sometimes you have to be "careful" about the signal voltage, and sometimes you might have to bias the AC audio signal so it can handle the negative voltage swing, etc., so make sure to research that carefully.
*There is something called a [u]differential amplifier[/u] that might work to (mostly) subtract-out the radio, leaving noise. That's similar to how noise canceling headphones work to subtract-out the noise. But it could be tricky because of the time delay and reflections/reverb in the acoustic signal.