The problem has to do with the map/sound file for the map. The best thing to do is delete the .bsp, and the sound file for the map, and reconnect to the server. When you reconnect you will automatically start downloading them again.
If that doesn't work, you will need to change your directX level. You can do that by right clicking on the game you want to change in this case DOD, and go to properties, click in set launch options and box should appear. I wouldnt suggest typing anything else in here other than what i am about to tell you, seeing as to how it is for people who know what they are doing. In this box type in "-dxlevel 80" (without the quotes) then click ok and restart steam....if setting it to 80 doesnt help, you can try one of 3 other options, either going higher or lower with the setting of 70, 81, or 90.
(Thanks Lungbuster for the DirectX settings, archives are your friend)