Setting conference_set_auto_outcall_flags=mute only sets the initial state to muted.  The attendees can still press 0 to unmute (if the key mappings are left at their default values).  So you can use send_dtmf to unmute the loopback extension, and then stream the audio.

 

E.g., expanding my code from earlier:

 

streamwelcomefile.lua
<code>
 
if ( session:ready() ) then
  session:answer();

  -- Must wait a couple seconds after connecting before sending DTMF,

  -- or it will get lost:
  session:sleep(2000)

   -- Unmute this attendee:

  session:execute("send_dtmf","0@200")

  session:sleep(1000)
  session:streamFile("ivr/8000/ivr-welcome.wav");
  session:streamFile("ivr/8000/ivr-thank_you_for_calling.wav");
  session:hangup("NORMAL_CLEARING");
end
 
</code>