That's what I figured, although I'm going to need a new headset before I go online.
Absolutely get a headset. I've started using mine for the first time ever, thanks to L4D. There are just too many times where not having one will cost your (or your team's) life. The characters are pretty good at calling out enemies or items, but pretty good isn't enough, and they don't call out hordes or help guide team mates that have been disoriented.