I don’t know of a product we currently carry that you could connect to the Edge and easily save audio data. You could potentially send low bitrate audio via serial to a Pi, but there are probably better options for this.
Since you already have a Pi, it might be easier to use that to record and store audio recordings. A [USB microphone and a thumb drive would do the trick. All you’d need is a bit of software running on the Pi to receive audio from the mic and save it to a file on the thumb drive. [This tutorial might be helpful for that.