I have a small USB speaker. It is on 99% of the time I am riding solo (which is 99.9% of my ride time). The only time it is not on is questionable winding narrow country roads where I feel hearing cars coming up behind me is safer. Can't say they'd sound different when they are about to hit me but gives me piece of mind.
My routine weekday rides are all rural, in the last year on those routes, I've seen maybe 3 other people on a bike total and not a single walker/jogger/runner or dog on a leash. The cows don't seem to mind the music.