MorphVOX can be used with all Valve games and Half Life mods, including Day of Defeat.
Day of Defeat uses the system default microphone for audio input.
If you have not set the Screaming Bee Audio driver as your default please click here for instructions on setting the driver before running Day of Defeat.
If the your voice is faint then you might want to click on boost microphone gain.
It is normal for the voice to sound distorted or "crackly" during the test. This happens even without MorphVOX installed, and the quality of the voice during the game is always better (don't ask us why).