This Extension will play random sounds from asterisk standard sound set forever. You might have to adjust the path uses. This example works on Debian:

exten => endless,1,Answer()
exten => endless,2,Set(RANDSOUND=${SHELL(ls /var/lib/asterisk/sounds/en | shuf -n 1 | head -n1 | cut -f1 -d"." | tr -d "\n")})
exten => endless,3,NoOp(*${RANDSOUND}*)
exten => endless,4,Playback(${RANDSOUND})
exten => endless,5,Goto(2)
exten => endless,n,Hangup()