My pet theory is that playing a musical instrument is less common a hobby among programmers now than it was a generation ago, because now programming tends to be associated with "Computer Science" as its own discipline, rather than a subdiscipline of math.

Near the end of this interview [http://tex.loria.fr/historique/interviews/knuth-clb1993.html], Don Knuth says that one way CS graduate students have changed since the 70s is that they are less interested in music:

"What changes have you seen in the students coming into the computer science program over the years?"

"Knuth: There is a very profound change that I can't account for. In the 70s, the majority of our students were very interested in music. The first thing we'd ask them when they came in was 'What instrument do you play?' We had lots of chamber groups and so on. Now almost none of the students are interested in music. I don't know if it's because a different kind of people are enrolling in computer science, or because it's true of all today's students, or what. If you ask computer science students now what their hobby is, the chances are most of them will say 'Bicycling'. I recently had one who played a harmonica, but there were almost no musicians in the group."