usually you can use stopsound (if you are using spk to play sound) or mp3 stop (if you are using mp3 play) to stop the sound before play the next next one.
PHP Code:
client_cmd(id, "mp3 stop; stopsound;") //Use to stop all sound
client_cmd(id,"stopsound") //Use to stop wav/spk sound
client_cmd(0,"mp3 stop") //Use to stop mp3