[System.Serializable]
public class Instrument {
    public AudioClip sound;
    public double freq = 1, offset = 0;
    public float amp = 0.6f;
    public bool mute = true;
    private double nextTime;
    private double tempo_spb;
    private AudioSource[] audio;
    private int audioIndex = 0;

    public void Init(GameObject gameObject, double startTime, double tempo_spb) {
        this.tempo_spb = tempo_spb;
        nextTime = startTime + offset * tempo_spb;
        // double buffering two AudioSources enable PlayScheduled without clipping old sound
        audio = new AudioSource[2];
        for (int n = 0; n < 2; ++n) {
            audio[n] = gameObject.AddComponent<AudioSource>();
            audio[n].clip = sound;
            audio[n].volume = 0f;
        }
    }
    public void Update(double time, double bufferTime) {
        audio[audioIndex].volume = amp * (mute ? 0f : 1f);
        if (time + bufferTime > nextTime) {
            audioIndex = (audioIndex + 1) % 2;
            audio[audioIndex].PlayScheduled(nextTime);
            nextTime += freq * tempo_spb;
        }
    }
}

public class Sequencer : MonoBehaviour {
    public double tempo_bpm = 140;
    public Instrument[] loops;
    public double bufferTime = 0.2;
    private double tempo_spb;

    void Start () {
        tempo_spb = 60.0 / (tempo_bpm * 4);
        double startTime = AudioSettings.dspTime + bufferTime;
        for (int n = 0, c = loops.Length; n < c; ++n)
            loops[n].Init(gameObject, startTime, tempo_spb);
    }
    void Update () {
        for (int n = 0, c = loops.Length; n < c; ++n)
            loops[n].Update(AudioSettings.dspTime, bufferTime);
    }
}