I was told this card only passes video with the HDMI and that for audio I would have to connect another cable to my sound system.
You were told a right and a wrong. Wrong in that you don't need to connect a cable to your sound system, you need to use the S/PDIF cable that should have come with your video card and connect it to the S/PDIF header on the video card and the S/PDIF header on your sound card (either onboard or actual card).
Right in that it does pass through audio over HDMI when the above cable is connected. Unless you card is an oddball from MSI from what I can see they all have the S/PDIF header on them, and they all came with that cable. Or at least the three here do, Newegg.com - Computer Hardware,Video Cards & Video Devices,Desktop Graphics / Video Cards,MSI,GeForce 9 series,GeForce 9400 GT
If all you are looking for for is audio over HDMI then you shouldn't have to spend anything, if you're looking for a card that will slap that 9400 GT every which way from Sunday go for that 5770.