How do I add to channels or sine waves?

I want to make it so I can play either beats or chords at the same time as the normal tones. However anytime I try it always ends up sounding irritating. Here is the music player script

 using UnityEngine;
 using System;  // Needed for Math
 public class Sinus : MonoBehaviour
 {
     // un-optimized version
     public double frequency = 440;//This detects key and 
     public double gain = 0.05;
     private double increment;
     private double phase;
     public PlayerController moveFor;
     private System.Random RandomNumber = new System.Random();
     private double sampling_frequency = 48000;
     void Start()
     {
         moveFor = GameObject.FindObjectOfType<PlayerController>();
     }
     void OnAudioFilterRead(float[] data, int channels)
     {
        
             // update increment in case frequency has changed
             increment = frequency * 2 * Math.PI / sampling_frequency;
             for (var i = 0; i < data.Length; i = i + channels)
             {
                 phase = phase + increment;
             // this is where we copy audio data to make them “available” to Unity
             data[i] = (float)(gain * Math.Sin(phase));
 
         
                 // if we have stereo, we copy the mono data to each channel
                 if (channels == 2) data[i + 1] = data[i];
                 if (phase > 2 * Math.PI) phase = 0;
             }
       
     }
     
 }