The External Input on the One works in the same way as our other instruments, but not in the same way as a traditional mixer. The signal is not routed directly to the Main Output. instead, the External Input is routed through the same signal flow as the oscillators; when a key is pressed it opens up the envelopes and allows the signal to pass through the Filter and Amplifier section in the synth.
While you do have to hold down a key or use the Hold button to get the sound to come through continuously, the benefit to this, especially on the One, is that you can then sculpt the External Input through both the SVF and Ladder Filter and the Envelopes, allowing for more interesting and complex sounds.
To get started using the Vocoder make sure the mic is plugged in and is getting signal in the One. You can check this by pressing the "MORE" triangle button in the Mixer section, then choose which input you are using, turn up the level and send it through one of the filters. If you don't want to hear the dry signal when using the Vocoder you can turn the level of the External Input down in the Mixer.
Next, press the "MORE" triangle button in the Synth Effects, press the soft button at the top of the screen over "EFFECT TYPE" and choose the Vocoder. Once you have chosen the Vocoder press the soft button above "EFFECT PARAMS" at the top of the screen to adjust the controls. Use the Mix control to set the level of the Vocoder signal (as you turn the knob clockwise you will hear more of the Vocoder) and the Sensitivity control to set the sensitivity of the Mic Input.