The vocoder splits both signals into multiple frequency bands (usually between 8 and 40 bands). It listens to how loud each frequency band is in your voice (Modulator) and tells the synth (Carrier) to get louder or quieter in those exact same bands. The result? The synthesizer "speaks" your words.
At its heart, a vocoder is a cross-synthesis engine. It doesn't just "add an effect" to your voice; it uses your voice to shape a different sound source.
"The output is too quiet."
This is typically a vocal track or a rhythmic sound like a drum loop. The vocoder does not "hear" the pitch of the modulator; instead, it analyzes its harmonic envelope and volume levels. The Carrier (The "Sound"):
