Even though humanity has yet to build a robot that can carry on a conversation with a human, we all know what a robot's voice sounds like: tinny, droning, with more than a hint of mechanical buzz in the background. Using the open-source sound editor Audacity, it is easy to make a recorded sample sound like a robot.
Record or open the voice sample you wish to alter.
Select the section of the recording you want to alter by clicking and dragging along the track window.
Select "Delay" from the effects window.
Set the "Decay" slider to around 10. This setting determines how quickly the echoes in the voice will drop in volume. A lower setting will result in a shorter, more buzzing echo, while a higher setting will be more vague and foggy.
Set the delay to near the lowest setting, preferably 0.01. This will cause the echoes to overlap one another very closely, causing a highly compressed set of echoes.
Change the number of echoes to a value between 20 and 50. More echoes will provide a harsher buzz to your robot voice.
Click "Okay." Listen to your sample.
Hold down "Ctrl+R" to apply the filter again. You will probably need to repeat this step at least a dozen times to get your voice sufficiently robotic.
It helps if the voice sample is fairly monotone to begin with. For this reason, it is easier to record your own voice samples rather than trying to manipulate an unrelated sound file.