Why is it a problem to tap a shoulder, if it results in immediate attention?
If you get irritated by yelling, then don't. I guess I don't see why a touch on the arm is a problem, to get their attention (now if they're refusing to take off the headphones after they're aware you need to talk to them, different story). I hate yelling in general, so even if I need to talk to my kids about something I've gotten used to walking into whatever room they're in and being almost within touching range anyway so I can talk in my normal voice. I have a child that gets so engrossed in reading that they might as well be wearing earphones--so I have to touch them to get their attention and then wait for a moment for them to snap back to the real world. :)
I also have found that my DD loves having times to zone out (easy for me to understand, I use my ipod for that too). Sometimes I need my space; I don't think that my kids should immediately have access to me whenever and wherever they desire to; I deserve some "time off" (except for real urgent things of course!). I didn't really respect my kids' need for that until fairly recently, but boy has that helped my irritation level AND their irritation level!
I often listen to ebooks and talking podcasts with one headphone off--but having to do that with music would really make me sad!