How to Convert a Webex ARF File on OSX (a Mac)
Webex has a Mac version of their player on their player website available for download:  

https://welcome.webex.com/client/T31L/mac/intel/webexnbrplayer_intel.dmg

The software will install, but (IMO) it doesn't create an application link in Finder or in Launch Pad.  I personally don't have any WebEx recordings I can use to test, but you can launch the player manually.  It's located in the user's library folder under:

~/Library/Application Support/WebEx Folder/500


I was able to launch the player by using Terminal and typing:

$ open ~/Library/Application Support/WebEx Folder/500/Network\ Recording\ Player.app


You could also open that folder in Finder and create an alias to put on your Desktop/Applications folder.



Convert the Audio

The kicker here is your are going to need another app to "grab" the audio.  You can use Audio Hijack or Soundflower.  You can also do it via hardware (less than $30) which will also give you a high quality USB audio interface that you can use for other purposes (I personally use this method for my iMac interface to my Bose Wave Radio).