I've never seen any documentation for possible values of the action property within a GET Envelope Audit Events response -- to the best of my knowledge, none exists. You might be stuck having to 'reverse engineer' to determine possible values. i.e., create envelopes via the DocuSign web console, act on them in a variety of ways (both as Sender and as Recipient) -- for example: Send Envelope, Void Envelope, Decline Envelope, Sign Envelope, View Envelope, Pass Signer Authentication, Fail Signer Authentication, etc. etc. etc. -- and then examine the corresponding action values that appear in the GET Envelope Audit Events response for those Envelopes.
Obviously this approach is not ideal, since someone fairly new to DocuSign won't necessarily know all of things that can be done with an Envelope, in order to trigger the various action values in the API response. Perhaps someone from DocuSign will see this post and chime in with a list of all possible values...or better yet, add this info to the API Guide one day soon.