I would probably stick with onboard sound since majority of them sound all right, unless you are just that picky with having high quality sound. Then, you'll also have to look into getting good speakers too. But yes, all the other hardware are relatively cheap. Cables, usually your motherboard will come bundled with enough cables for two PCs, lol. I never use all of them and end up giving them away for throwing them out!!! Here are some other parts you may be interested in.
Only $30, and comes with software for burning.
LITE-ON 20X DVD+R 8X DVD+RW 8X DVD+R DL 20X DVD-R 6X DVD-RW 12X DVD-RAM 16X DVD-ROM 48X CD-R 32X CD-RW 40X CD-ROM IDE DVD Burner included extra White bezel - Retail
Case fans are cheap too. They range in price depending on size and features anywhere from $3.00-$20.00.
For the CPU heatsink/fan, I would just use stock since it comes with it. It's easier that way.
If you must get a sound card, try getting the new chipset (X-Fi):
You can browse to see which one you want. I like sound cards that have higher sampling rates, but they cost more.