If nothing is happening, go into console and type 'voice_enable 1' (without the inverted commas), that will definitively turn it on, if not make sure your speakers are working andthat the receive volume is turned up to full.
Ok, it might be a read-write error. So try right clikcing on the day of defeat source folder and clicking properties, then if the read-only tick box is shaded (even slightly) click it until it is completely white. Then click apply and say to change the rights on all files and subfolders, this should make sure that any .cfg file that was read only is now writable and just try to do the console thing again.
Or else full uninstsall of DoD:S and delete the DoD:S folder in the steamapps folder.