Wow, I swear someone just PM'ed me this exact same question - oddly, in almost the exact same words.
The easiet way to use a mic while playing a steam game is to use the built in feature. What you have been semi unclear about is your exact problem.
For a generic sence - in order to use a mic in game, you will have to check your windows sound settings. Controle pannel > sounds then make sure to set your recording device to your mic. I find it the most usefull to open the sound pannel, click on file > recording and then put the check box under your mic. Here you can also adjust the mic volume.
Note: when you first open the sound pannel - it is in playback mode and adjusting the volume of the mic here will ONLY adjust the sound at which you hear your own mic, NOT the volume others will hear you. It is common to turn this volume down and the recording volume up so that other hear you but you ignore your own voice.
Then once DOD is open - make sure to set your 'voice' button under the controls section, by default it is usually 'k'.
Also note - If you own n audigy 2 - or cirtain other cards that have two mic holes, and you are using mic #2, steam will auto change your recording device to mic#1. To get arround this there is a command parameter you can add to your exe file however I havnt memorized it and wont look it up unless someone actually needs it. You could always look it up yourself too
Luckily I'm a nice guy.
