Enable Audit on ALTER USER

I would like to enable audit on ALTER USER to audit all alter user commands.

I have enabled with following steps, however, i still dont see alter user details

I am using the custom procedure(similar to mark's procedure as per below thread)  to alter the user to unlock user account by support engineers. We have huge no.of end users accessing applications, so, we get many end user request to unlock their user account.

I have created the custom procedure as SYS user to unlock the user account and provided EXECUTE privilege to support engineers. They are able execute the procedure which will ask for end user account to unlock.