DeepSound is a steganography tool which can conceal confidential data within WAV or FLAC files (and encrypt it, too, for good measure).
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.
And finally, 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.
There are a few small issues and irritations here; the interface is a little on the basic side, for instance. But overall DeepSound works well and is easy to use. Check the documentation page, for instance, to see the very few steps involved in making all this work.