So you have a working conference bridge in asterisk and you want to play a audio file in the conference bridge. Here is how you can add audio to the conference bridge.

[ConferenceAudio] ; Users would join the conference at extension 5000
exten => 5000,1,Goto(start,1)
; Trigger audio playback with extension 105
exten => 105,1,Originate(Local/inject@ConferenceAudio/n,exten,
; Users join the conference here
exten => start,1,NoOp()
same => n,Answer()
same => n,MeetMe(31337,d)
same => n,Hangup()
; Use a couple flags to quietly enter the conference
exten => quiet_join,1,NoOp()
same => n,Answer()
same => n,MeetMe(31337,dtq)
; This triggers the file to be played into the conference
exten => inject,1,NoOp()
same => n,Playback(silence/1&tt-weasels)
same => n,Hangup()