Irregular sound bug

I have a sound bug when I play Minecraft. If I have an other sound playing (a video on youtube, a MP3, ...), the sound become a little bit scratchy and noisy. If i set the master volume at 0% in Minecraft, there are no more problem. I already try to re-download Minecraft, try snapshots but always the same problem. I think it links to new divided controls of sound on Minecraft.
I hope my description was enough clear (i don't speak english very well)
Thanks !