...
var handle_intro_played = media_play("demo.ogg",NULL,100); // play your stream
while(media_playing(handle_intro_played) < target_frame_number) { wait(1); } // keep playing until target_frame_number has been reached
media_pause(handle_intro_played); // pause the first stream
var handle_next_mediastream = media_play("demo2.ogg",NULL,100); // start next one
...