We all know how annoying flights can be with row mates who want to talk through the whole flight, those that snore loudly, children, 20 somethings off to spring break, etc. Also, if you are going to a city destination or party destination, or stay in a hostel anywhere, it can be noisy.
One of the most important things I bring with me on a trip is ear buds, and ear plugs. Noise cancellation headphones/earbuds are a wondrous thing and worth the investment. But just in case they somehow fail (break) I always have some ear plugs to muffle the sounds around me. Especially for when I want to sleep, I find it easier to sleep with ear plugs in than ear buds or headphones on. So keep a pack of ear plugs in your carry on and while you’re at it toss in an extra pair of cheap ear buds. I know if my buds failed me I’d die without music, so crappy ear buds are better than no ear buds!