norns passthru example

Engine_SimplePassThru.sc

Engine_SimplePassThru : CroneEngine {

	var amp=0;
	var <synth;

// this is your constructor. the 'context' arg is a CroneAudioContext.
// it provides input and output busses and groups.
// see its implementation for details.
	*new { arg context, doneCallback;
		^super.new(context, doneCallback);
	}

// this is called when the engine is actually loaded by a script.
// you can assume it will be called in a Routine,
//  and you can use .sync and .wait methods.
	alloc {
		//Add SynthDefs
		SynthDef(\passThru, {|inL, inR, out, amp=0 |
			// read stereo
			var sound = [In.ar(inL), In.ar(inR)];
			Out.ar(out, sound*amp);
		}).add;

		context.server.sync;

		synth = Synth.new(\passThru, [
			\inL, context.in_b[0].index,			
			\inR, context.in_b[1].index,
			\out, context.out_b.index,
			\amp, 0],
		context.xg);

// this is how you add "commands",
// which is how the lua interpreter controls the engine.
// the format string is analogous to an OSC message format string,
// and the 'msg' argument contains data.

		this.addCommand("test", "ifs", {|msg|
			msg.postln;
		});

		this.addCommand("amp", "f", {|msg|
			synth.set(\amp, msg[1]);
		});
	}

	free {
             // here you should free resources (e.g. Synths, Buffers &c)
// and stop processes (e.g. Routines, Tasks &c)
            synth.free;
	}

} 

passthru.lua

-- PassThru Test

engine.name = 'SimplePassThru'

local audio = require 'audio'

function init()
   print("Simple Pass Thru Test")
   audio.level_monitor(0)
end

function key(n,z)
   if n == 2 then
      if z == 1 then
	 screen.move(10, 10)
	 screen.text("Listen")
	 screen.update()
	 engine.amp(1.0)
      elseif z == 0 then
	 screen.clear()
	 screen.update()
	 engine.amp(0.0)
      end
   end
end

audio.monitor_off() is not a valid method of audio.

apologies; that's right, the method was removed in 2.0 and this gist is pretty old. updated