My fifth-grader is just finishing her first year in band. Our school year started much the same way; the first time the band director introduced himself to us was at the December concert!
I was ready to be more involved, so I made a point of introducing myself, emailed regularly, and finally just told him I was coming in, and asked when he was available. We've had a couple of great meetings since then, and I'll be going in twice this week to help out with the end-of-the-year ice cream social. I'm also planning to help out with summer band camps.
My guess is that sometimes parents offer to help, and then, for whatever reason, don't end up sticking around. It's seemed to me lately that I need to prove myself by showing up and being steadily engaged before people will really take me seriously. Also, keep in mind that often these teachers' strong suits are music and education, and they may not have the talent, energy, or time for managing volunteers.
My goal, as the program matures, is to get and keep the parents involved from day 1. It will be a slow evolution, I'm sure, but one that I hope will strengthen the music program throughout the entire district.