DeepSound is a steganography tool which can conceal confidential data within WAV or FLAC files (and encrypt it, too, for good measure), and even audio CD tracks.
The program includes a simple built-in browser which allows you to locate and choose a suitable carrier file.
Next, you're able to add whatever files you'd like to hide within the carrier.
DeepSound can embed that data, and optionally protect it with 256-bit AES encryption, exporting the results as a modified WAV or FLAC file. This will play as normal, so look to everyone else like a regular audio file, but only yourself (or anyone else who knows the secret) will be able to extract the secret documents.
Better still, if you burn the WAV or FLAC files to an audio CD then the program should still be able to retrieve the hidden documents later.
We say "should", because you have to make sure that your disc burning program doesn't process the audio in any way. In Windows Media Player, for example, we had to click "More burn options" and disable "Apply volume levelling across tracks". You'll similarly need to turn off normalisation elsewhere.
But once our preparations were complete, everything else ran very smoothly. We selected a carrier FLAC file, embedded a PDF in it, and exported it as a WAV. This was burned to an audio CD with our reconfigured Windows Media Player. And if we opened that audio CD track in DeepSound later, and entered our password, we were able to see and extract the hidden file.
There are a few small issues and irritations here; the interface is basic and less than intuitive, for instance. But overall DeepSound works well, and, once you understand the basics, is easy to use. Check the documentation page, for instance, to see the very few steps involved in making all this work.