No offense to Jim Robinson, but don't get a CD, as its (very) easy to get confused about who, when and were things happened (that happened to me last time I was there). If at all possible, hire a guide--either as part of a group tour or if you can afford it, one for your family. The park service has both kinds, and I think even private guides are available.
Don't be surpised to almost feel the vibes of the place...as parts of Gettyburg's battlefield were incredible killing fields. Some have reported camera malfunctions and other unexplainable phenomena in places where men fell so thickly the bodies were nearly on top of each other. I know for myself, if you had dropped me off blindfolded there, I would know it wasn't a normal place.... there's something intangible that makes one sense something very serious happened there.
Oops, I misread. That was supposed to be "John Robertson," not the name of our illustrious host, and really should have been in response to "lil Freeper" who had suggested the CD.
I think group guides can be had for $20 or $30 dollars...and a CD is $10 or $15. You'll get a lot more from a real human than a CD.
Take care....
My son and I took the audio tour last year. Amazingly both our camera batteries died at exactly the same time -- at Little Round Top.