Hi,
OR, do it in hardware with more channels available. See BlipTronics shield: http://www.bliptronics.com/item.aspx?ItemID=116
This has 7 bands of frequency with separate outputs. This is used in some of the high-end stereo systems for the Oh-Cool! display.
You could buy the chip, and make you own.. See: http://www.bliptronics.com/item.aspx?ItemID=116 and discussion: http://arduino.cc/forum/index.php/topic,8328.15.html