This notice is included because I got a lot of email from visitors who were looking for recordings of something, and were puzzled about why my site "didn't work". After adding this notice, such email dropped to nearly zero. So musicians will have to tolerate seeing this notice every time they drop by to find a tune.
The basic problem is that the English language has no effective way to distinguish music (something you put in a machine and listen to) from music (something you put on a music stand and read while playing it on your instrument).
This problem is also why this site exists. Using the big search sites to locate the readable kind of music was very frustrating, because the results were always buried in 1000 times as many matches for sites trying to sell me recordings. What was needed was obviously software that understood the difference and could return only the kind of music that musicians wanted.
This is currently only implemented for ABC music notation, because it was the first out that was amenable to analysis by software. There's also readable music online in the form of PDF and JPEG images, but extracting things like titles and keys from those is not really feasible. MIDI is barely feasible, and also has the problem that turning it into readable music is nontrivial.
Someday soon, I may also be able to return LilyPond and MusicXML files. As soon as there is a significant amount of music online in those forms, I'll dig into indexing them.