public PayloadBuilder sound(final String sound) {
        if (sound != null) {
            aps.put("sound", sound);
        } else {
            aps.remove("sound");
        }
        return this;
    }
Sets the alert sound to be played.

Passing {@code null} disables the notification sound.

@param sound the file name or song name to be played
             when receiving the notification
@return  this
src/main/java/com/notnoop/apns/PayloadBuilder.java