I have a PCI Creative X-fi sound card - it's one of the first XFi cards - extreme gamer or something. You can't even buy one anymore unless off ebay I suppose...Anyway, I was about to clean out my case today and I heard a small "clank," and there I saw the heatsink off my Creative card lying hehe...It's really small - no fan, just the metal heatsink, maybe 1.2 in square at the most. Will I be alright without this thing? Or do I need to figure out how to get it back on? Or buy a new card? Or just go with the onboard...

I doubt you'll run into issues. Heatsinks on modern audio chips are superfluous, really just there for show. Like heatspreaders on most RAM.

Or do I need to figure out how to get it back on?

If it were me (i'm a bit persnickety in these regards)... I'd glue it back on with some thermal "epoxy" adhesive like this Arctic Silver Alumina Thermal Adhesive or their Premium Silver Thermal Adhesive. Heed the warning application about it being used "When it absolutely needs to stay on forever."

Well in the end I bought some Arctic silver epoxy off amazon... that should fix it!