We jsut had a post on this subject recently. ALOT of people got a little irrate about people not having there kids wear headphones. I was amazed at how bothered peopld actually got. I would have your child start practicing. My DD2 likes to watch DVDs in the protable player sometimes, so when she does, I put headphones on her. She usually keeps them on, but sometimes the yfall off, as they are a bit too big, but she is usually standing and moving when it happens. I don't see a problem when on the plane for us. You can also just get her used to wearing tehm, or no sound when it is on if she won't wear them. Some kids don't mind if there is sound anyways.
I really don't care too much what you do, but since I found out how many people get mad, I will do my best to keep noise ata minimum. My biggest fear is that she will just have a bad day or something and decide riding a plane isn't in the cards and just cry for no reason. This doesn't happen often, but it can here or there!