Hello, May I request some help?

I've been editing a video and then I used Windows Movie Maker to combine the video and audio but every time I press the play button, it keeps getting stuck at the beginning, I just ignored it and I pressed save movie, after that I clicked youtube and saved. Once it was finished when I opened it, it was faster than I expected. My question is that how do I keep the video and audio in it's original state?

*By the way the video was made by a program called MikuMikuDance