I'm looking into playing around with procedurally generating music. I'm hoping to find a really a simple API where I can just call out instrument, note, duration and string together a song (I'll take anything of course, but that would be my preference). Is there any library that does this?
Your best bet is a music programming environment, of which there are several.
Csound is one of the best known ones. Here is their website.
Max MSP is also another widely used option, and it provides a visual programming interface too. It is, however, commercial.
Another well known option (and widely used by experimental electronic musicians) is SuperCollider. This is its webpage.
Here's a Wikipedia article describing similar languages/environments.
You can also use a general programming language with the right libraries to do audio/music work. Java, for one, provides the Java Sound API.