Although Samsung and Google have not officially released this "official" volume control bug fix for the European Galaxy Nexus, the folks at MoDaCo have leaked it for early adopters and extra testing in the wild. You have to root your phone (which will wipe it) to get this one to work, so follow the instructions at the MoDaCo source link carefully. The problem is that the volume fluctuates when the 2G 900MHz is being used. Although this appears to be a hardware issue, this leaked fix seems to indicate that it is fixable with just a software patch.
On an interesting side-note, there is talk from some retailers that Samsung is recalling and halting shipments of the Galaxy Nexus to resolve the issue, although this has not been confirmed. This would be strange if it is fixable with a software patch. If you are one of the folks that is experiencing this issue, and try out this fix, let us know in the forums how it goes.
Source: SlashGear and MoDaCo