Usually it doesn't matter much which is slave and which is master, as long as you keep hard drives and CD/DVD drives on different IDE channels. I always make CD/DVD writers master and CD/DVD readers slave to be on the save side, but this is more a habit of me than a real decision.
The audio cable you found in your package is (most likely) the cable to connect your CD/DVD drive to your soundcard. It is for analog audio, and is a relic from the past; it has little use these days. I don't use one and so far never missed it. I wouldn't use it to connect two drives with eachother however; I don't know if there is a chance of short circuiting, but it just isn't what it is ment to do.