Does music have any intellectual content?

As opposed, I suppose you mean, to affective or emotional content? Yes, both in the lyrics or librettos of various musical compositions and in the web work of meanings that have come to be attached to various sounds. Like most, if not all, artforms, music exists in an historical context, and within any context music relates to other music. So rhythms, harmonies, instrumentation, chord progressions, intonation, etc. evoke symbols, social ideas, abstract ideas concerning music theory, social criticism, human relationships, the divine, etc. Musical compositions themselves are associated with cultural movements (modernism, tradition, militancy, rebellion), fashion, politics, even entire civilizations (the Europe, Africa, India). When we listen to a song, we listen to a history and to a society.

What is music? I can recognise music from cultures other than my own as being music, even if I don't enjoy it; but what makes a series of sounds 'music'? Similarly (I assume), when does human vocalising become song?

What a fascinating question. I hope that some of my co-panelists can give you the answer this question deserves. For myself, I would briefly and cautiously answer this way: What makes a series of sounds (or even a single sound or even a silence) music is our agreement to consider it as music. Just as John Cage invites people to consider the silence and random sounds that occur during a 4 minute and 33 second period music (his piece is called, 4' 33" ) and Marcel Duchamp invites people to consider a urinal as sculpture (he called the piece Fountain ), we make something music when we interpret it as music.