Some notes don't play a sound.

sampler.swift

import Foundation
import AVFoundation
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

// NOTE: ~/Documents/Shared Playground Data/GeneralUser GS v1.471.sf2
// http://schristiancollins.com/generaluser.php
let url = PlaygroundSupport.playgroundSharedDataDirectory.appendingPathComponent("GeneralUser GS v1.471.sf2")

let engine = AVAudioEngine()
let sampler = AVAudioUnitSampler()

engine.attach(sampler)
engine.connect(sampler, to: engine.mainMixerNode, format: nil)

try engine.start()

try sampler.loadSoundBankInstrument(at: url,
                                    program: 12,
                                    bankMSB: UInt8(kAUSampler_DefaultMelodicBankMSB),
                                    bankLSB: UInt8(kAUSampler_DefaultBankLSB))

var playingNotes = [UInt8: Timer]()


func randomNote() -> UInt8 {
    let scale = [0, 4, 7, 9].randomElement()!
    let octave = [-12, 0, 12].randomElement()!
    return UInt8(60 + scale + octave)
}

func randomNotes() -> [UInt8] {
    let count = (1..<4).randomElement()!
    var notes = [UInt8]()

    while notes.count != count {
        let note = randomNote()
        if !notes.contains(note) {
            notes.append(note)
        }
    }

    return notes
}

func playSingleNote(_ note: UInt8, _ duration: Double) {
    if let timer = playingNotes.removeValue(forKey: note) {
        timer.invalidate()
        sampler.stopNote(note, onChannel: 0)
    }

    sampler.startNote(note, withVelocity: 127, onChannel: 0)

    playingNotes[note] = Timer.scheduledTimer(withTimeInterval: duration,
                                              repeats: false,
                                              block: { [weak sampler] timer in sampler?.stopNote(note, onChannel: 0) })
}

func playNotes(_ duration: Double) {
    let notes = randomNotes()
    notes.forEach { playSingleNote($0, duration) }
    print("Notes: \(notes)")
}

let tempo = 120.0
let duration = 60.0 / (tempo * 2.0)

Timer.scheduledTimer(withTimeInterval: duration,
                     repeats: true,
                     block: { _ in playNotes(duration) })