Try this:
;(901): Room added by system
exten => 901,1,Set(CHANNEL(language)=en-US)
exten => 901,n,Set(CDR(accountcode)=1)
exten => 901,n,Answer()
;exten => 901,n,Set(CONFBRIDGE_JOIN_SOUND=beep)
;exten => 901,n,CONFBRIDGE_MOH=default)
exten => 901,n,Set(CONFBRIDGE(user,music_on_hold_when_empty)=yes)
exten => 901,n,Set(CONFBRIDGE(user,music_on_hold_class)=default)
exten => 901,n,ConfBridge(${EXTEN})
exten => 901,n,Hangup