Oh, absolutely. Earliest one I'm aware of is Bach's "Art of the fugue", where he embedded his name as a third voice.
It's not quite on the level of embedding images in a spectrogram, but buried deep in Amarok by Mike Oldfield is morse code for "FUCK OFF RB", a reference to Richard Branson, owner of his label, Virgin Records.
Re:Fez soundtrack contains hidden data
by EmmanuelOga on Friday April 20, 2012 @09:34PM
(#3870542)
It is pretty easy to accomplish too. Here is a perl program an explanation on how to encode images on sounds.
for some bizarre reason, i was interviewed about this. in 2002.
http://www.bastwood.com/?page_id=10
There must be precursors of the practice before the advent of computer music, or even digital/electronic music production altogether, but I can't recall any; I'd love to be pointed to them.