It depends...but generally I find it annoying/rude when people wear earphones while playing, unless they are playing by themself. The people that play with earphones during a tourney are not paying attention to other people which annoys some, but in some sense is part of what disc golf is really about. You vs. the course, not you vs. them. There is no rule against it, but they are not making any friends either. They are focused on their game and not being distracted by others.
I will sometimes listen to my ipod w/ earphones when I practice mindless stuff by myself, but I also like to hear my surroundings and be aware of what is around me. I would not wear earphones at some courses due to the neighborhood and likely hood of being jumped again.
I also sometimes listen to the speaker on my ipod which is hardly loud when I'm practicing or just playing a round with a friend that I know likes my music.
When I play any tournaments, singles, doubles, triples I never listen to music. I think earphones would totally defeat the purpose of playing double or triples. I like to know what is going on around me, but it doesn't distract me. I can get into a zone just playing a song in my head and block out people, although my putting still sucks. I want to be able to hear that fore! Although if I get stuck playing with somebody annoying the hell out me again, I may have to pull out the earphones. I noticed a few pros wearing earphones at the 2008 worlds, but I wonder if that may have been out of boredom from playing all week.
I work with this kid(22yr) (man i feel old saying that now im almost 30)who wears his earphones often and it annoys the hell out me. I have to repeat almost everything I say because he can't hear the first couple words. It takes him extra time to do stuff because he has to wire up or wire down to do certain things. Not to mention the severe ADHD he has. He's already deaf enough, and I can hear his music as if I had his earphones on. I almost threw a socket wrench at him to get his attention one time because he couldn't hear me yelling at him, and I was about 20' high on a scaffold and needed something. I call him Meatwad now after the ATHF episode with MC Pee Pants! lol.