A simple procedural audio (pure sine wave) generator for Unity, using OnAudioFilterRead().

SimpleAudioGenerator.cs

using UnityEngine;

// MB: Simple procedural audio generator.
// Generates a pure sine wave/tone at a given frequency.

public class SimpleAudioGenerator : MonoBehaviour
{
    int outputSampleRate;
    const float TAU = Mathf.PI * 2.0f;

    // Continous phase = continuous tone.

    float phase = 0.0f;

    // Careful, this can be very loud!

    [Range(0.0f, 1.0f)]
    public float amplitude = 0.5f;

    // ~Typical range of human hearing is 20Hz to 20kHz.

    [Range(100.0f, 8000.0f)]
    public float frequency = 440.0f; // A4.

    float phaseStep;

    void Start()
    {
        outputSampleRate = AudioSettings.outputSampleRate;
    }

    void Update()
    {
        phaseStep = (frequency * TAU) / outputSampleRate;
    }

    void OnAudioFilterRead(float[] data, int channels)
    {
        for (int i = 0; i < data.Length; i += channels)
        {
            float sample = Mathf.Sin(phase) * amplitude;

            for (int j = 0; j < channels; j++)
            {
                data[i + j] = sample;
            }

            phase += phaseStep;

            // Phase = [0.0f, TAU).

            if (phase > TAU)
            {
                phase -= TAU;
            }
        }
    }
}