Might want to hold off on buying a Galaxy Nexus for the moment

cgk1

Now that people are buying this device, a number of users are reporting a serious hardware fault - it seems that the radio is not properly shielded and when the phone makes use of a 2G signal it will also also turn down the volume. The upshot of this is that the phone will randomly go to silent mode, so you may be unable to hear it ringing or if you are in a call the person on the other end of the phone.

This appears to be a hardware fault and not something that is fixable by a software update. Until google/samsung make an official comment on this might be best to hold off.

Some reports of this:

http://crave.cnet.co.uk/mobiles/why-we-cant-recommend-the-samsung-galaxy-nexus-yet-50006175/

http://code.google.com/p/android/issues/detail?id=22021

http://forum.xda-developers.com/showpost.php?p=19493968&postcount=1