Well, I can hear them. It's only in one ear (the one facing away from the road), and lectures aren't particularly noisy anyway. If I were wearing noise isolating in-ear headphones in both ear and listening to heavy metal at a blasting volume, yeah, I'd agree that there'd be a danger, but as it is, I have no problem hearing them a long ways away and keeping an eye on them in my mirror. Usually a head turn as well just clearly to signal that I see them (if I just use the mirror, I think a lot of people just think I'm oblivious and unaware of their presence, as most aren't aware of/don't know what that contraption attached to my glasses is, and assume I'm blind behind me).
Lectures are Zen for me, I get to learn a lot about the world during my bike rides. I would indeed prefer nothing to some generic blasting music that doesn't really improve me in any way, but I prefer the lectures to nothing. I'm more into the visual of biking anyway, the natural beauty around me. I used to think this place was ugly before I started biking, just a bunch of nondescript trees whizzing by you at 65mph that you watch through dirty, tinted windows while your on your way to somewhere important. On my bike rides, well - there's so much that just takes my breath away. I've practically become a neo-luddite, I love biking the undeveloped and agricultural areas, any dense human settlement just looks like an ugly, grey, dreary pox on the land.