dietermoreno has really no idea how a radio works, but his suggestion that the audio has to get to the audio amplifier's input is correct.
But I think you already know that
You need to determine where the audio signal(s) are in your device and connect them to your amplifier.
That may be more easily said than done (because we assume there's no audio output, right?)
If you can get some good images of the inside of this device, and if we can identify the chip(s) used, we might be able to suggest where audio can be grabbed from.
If you're *really* lucky, you may be able to wire them to the socket currently used for Audio input (which would be superfluous for this application) and make it the audio output.