Why is the sound of my YouTube music video bad and bouncy???

I have made a music video using windows movie maker. I uploaded my pictures with my songs. After that i saved it to my computer choosing best quality (recommended option). I viewed it back and it sounded great!! After i uploaded it to youtube and it was fine first but then after 10 minutes the sound started to get bouncy as if the music was getting both louder and quieter.. I tried removing the video and uploaded it again but the same problem appears.. I dont get whats going on as in my computer it sounds perfect! Any ideas?? Thanks!!!