That is exactly what we were thinking, 142victoria. It is most likely either a ground isolation issue or possibly a "noisy" power supply.
There are a couple of things you could try and identify where the issue is originating from. First, try powering just the Papa Soundie via USB and connect some headphones to the 3.5mm jack on the board. Listen to the tracks through the headphones and see if you hear the "whine"/interference. If you do hear it there, it might be something to do with the audio files themselves. This is the least likely thing but it's the easiest to test.
The next step would be, as 142victoria suggests, to make sure you have isolation between your power supply. Here's a direct link
to that post.
It might also just be a noisy power supply that is causing the issue here, too. If you have another way to power the circuit, give that a try and see if that helps reduce the noise here.
I hope one of these suggestions helps you resolve the issue here. If you still have issues after trying these suggestions, please take a few photos of your boards and circuit and attach them to your reply and we can continue troubleshooting.