Google and Samsung have confirmed that some units of Galaxy Nexus sold in the European markets were affected with a bug that caused fluctuation in volume levels.
Google is working on a software update that is expected to be out in a couple of days.
MoDaCo has released some ROMs which are said to the same ones being tested by Google internally.
These builds are unofficial and installing them would wipe out all data on your device. So, play around with them only if you know what you are doing!
For regular users, we suggest waiting for the official update from Google which should not be that far away.