Wow, impressive feature this is!
I am playing with the libraries and I succeeded encrypting and decrypting an mp3 file following the instructions 
I hoped that @BenMS would add in his OP that you also need to install libsodium before you do anything.
The instructions are from another thread here :
" 5.) install libsodium if you have not yet done so
Unfortunally, there are not libsodium packages for Ubuntu. So, you should download, build and install libsodium as follows:
Download the newer tar archive from https://download.libsodium.org/libsodium/releases/17 (you can also get the git repository from https://github.com/jedisct1/libsodium6).
Follow the ritual:
$ tar xzf libsodium-1.0.3.tar.gz
$ cd libsodium-1.0.3/
$ ./configure
$ make
$ make check
$ sudo make install
Once you have installed you have to reload/rebuild the GNU linkers. A simple sudo ldconfig is enough:
$ sudo ldconfig
Now run
$./configure
and it should generate the makefile. (I’m assuming you have at least build-essentials package installed)."