How to Create Database Audit Specification for a column?
I am not sure you can. However it is a bit confusing, MSDN says that it is possible:

http://msdn.microsoft.com/en-us/library/cc280645(v=sql.105)

However following the example that they give I get the following error:


  Msg 33211, Level 15, State 1, Line 0
  A list of subentities, such as columns, cannot be specified for entity-level audits.


The message above seems to be pretty definitive, I have tried their example on 2008, 2008 R2 and 2012 and I get the same message every time.