To make your voice sound robotic in Audacity, apply the program's vocoder effect to it. A vocoder, from voice encoder, uses a source -- your voice, in this case -- to modulate the frequency of a carrier sound wave. Audacity's vocoder uses the left-hand audio channel as the source and the right-hand channel as the carrier. To make your voice sound like a robot's in Audacity, you must record your voice in stereo, split it into two channels, then replace the right-hand channel with a sound wave.
Connect the microphone to your computer. Start Audacity. Click "Edit," then "Preferences." Click "Devices" in the left-hand column of the window that pops up.
Select "2 (Stereo)" from the "Channels" drop-down menu in the "Recording" section. Click the "Device" menu and select "Microphone." Click "OK."
Click the "Record" button. Speak into the microphone. Click "Stop" when you're finished recording your voice. Note the duration of the recording at the top of the waveform.
Click the "Audio Track" drop-down menu next to the recorded audio's waveform. Select "Split Stereo Track" to split the waveform into two channels.
Double-click the bottom audio channel. Press the "Delete" key to erase its contents.
Click "Generate," then click "Tone." Select "Sawtooth" from the "Waveform" menu and type "220" into the "Frequency (Hz)" box. Type the length of the recording into the "Duration" box. Click "OK."
Press "Ctrl" and "A" simultaneously to select both audio tracks. Click "Tracks," then "Mix and Render" to combine the two channels into one stereo track.
Open the "Effect" menu and click "Vocoder." Click "OK" to apply the vocoder effect and make your voice sound robotic.
Tweak the vocoder settings to customise your robot voice. Experiment with different types of sound wave to produce different effects.