Recently, we found out that the Samsung Galaxy Nexus had a volume bug which made it change its volume continuously when placed next to another phone. However, an official update is in the works. If you just can't wait for the official update, then you might want to check this out. This is an unofficial fix for the volume bug, follow the instructions and you will hopefully be able to resolve the issue on your device. Of course, this is an unofficial fix so please do at your own risk.