Yes, as others have mentioned, this is steganography, not cryptography. Basically, encoding a second message inside, in this case, a piece of music. If we want to talk about cryptography, we could say this resembles more simple, classic substitution ciphers.

Now, about encoding messages in music... I think there are many more interesting possibilities. For example, instead of words, you might encode more music itself. Digital music formats specially offer quite a lot of new ways to write 'secrets' in your music. Pitch and rhythm are related [1], and even though current digital music formats are not designed for this, you could technically write songs with other songs inside, at very different time scales. Surely someone will write hidden tracks like this in the future. Other possibilities could include encoding messages using specific frequencies (for example, inaudible frequencies, but not necessarily), which would be fun because you can encode messages in more traditional ways, not associating notes and lengths to letters, but numbers from specific frequencies (I think this would be currently feasible as long as files are not compressed/modified), so you could translate to more traditional, actual cryptography. Sorry if I went a bit off-topic, but I find all this so interesting!

[1] https://youtu.be/_gCJHNBEdoc super fun and educative video by Adam Neely, his work is worth checking out!