First of all, I am quite sure, headphones are illegal to use in California.. Within cars, bikes, motorbikes and any legal vehicle; which includes bicycles. Plus I think It would distract from the ride and its safety..
I do see motorbikes with open air speakers. Even though I love my kind of music; when motorbikes go by with music I don't like; I feel very annoyed. Not sure how music I like would effect my cycling. I do miss my kind of music, when I commute by bike over car. But still would not miss out on that bike commute.
To show my dedication to my cycling. On Sunday A.M, on my favorite radio station, they play 4 hours of 'accoustic music.' Great show. Used to stay home to hear it. But of late, I miss out on my favorite music for my bike ride. Would I like it on an open air speaker on my bike.
Like the sound of nature, when I do my Sunday moring ride on on my favorite crest of hills. Guess I would still choose nature over the music I like so well.