You can use the internal polyphonic synthesizer of Vocoder to create a vocoder effect without an additional carrier signal.
Prerequisite
Your project contains an audio track with audio material that is suited to serve as a modulator source; for example, a voice sample.
Note
The modulator source can be audio material from any audio track. If your audio hardware allows for a low input latency, you can also use a live audio source routed to an audio track.
Procedure
Result
When playing the modulated synthesizer signal via the MIDI track, a typical vocoder effect is created.