Automator action to adjust volume on audio files
How about something like SoX (free/donationware). It's features page lists wide variety of volume/level effects, including:


  
  Volume/level effects
  
  ...
  ...
  vol: Adjust audio volume
  
  


According to the SoX documentation, the following should double the volume of input.wav and write it to output.wav (the value after the -v flag is a multiplier)

sox -v 2.0 input.wav output.wav

You should be able to use this in an Automator workflow using the "Run Shell Script" action.