So it seems like using the Mixer object along with wave files should let us play the tones simultaneously. Now we need to get some musical note wave files, how can we do that?

I heard you like python so I made some wave files with python to play with circuitpython.

As I was on the search for wave files of tones I found myself wondering if we could use cpython to generate wave files. And of course we can! The code in this stack overflow answer gets us most of the way there. We can create some loops to generate each tone for us and save it into a wave file, perfect! I've got it set up to output 3 full octaves, but no sharps/flats currently.

#!/usr/bin/python
# based on : www.daniweb.com/code/snippet263775.html
# Adapted from sample code in Stack Overflow answer: https://stackoverflow.com/a/33913403/507810
import math
import wave
import struct

# Audio will contain a long list of samples (i.e. floating point numbers describing the
# waveform).  If you were working with a very long sound you'd want to stream this to
# disk instead of buffering it all in memory list this.  But most sounds will fit in
# memory.
audio = []
sample_rate = 8000.0 # 8k sample rate for "low quality".
                     # 44.1 kHz sample rate for "HD"

TONE_FREQ = {
    'C3': 131, 'D3': 147, 'E3': 165, 'F3': 175, 'G3': 196, 'A3': 220, 'B3': 247,
    'C4': 262, 'D4': 294, 'E4': 330, 'F4': 349, 'G4': 392, 'A4': 440, 'B4': 494,
    'C5': 523, 'D5': 587, 'E5': 659, 'F5': 698, 'G5': 784, 'A5': 880, 'B5': 988
}


def append_silence(duration_milliseconds=500):
    """
    Adding silence is easy - we add zeros to the end of our array
    """
    num_samples = duration_milliseconds * (sample_rate / 1000.0)

    for x in range(int(num_samples)):
        audio.append(0.0)

    return


def append_sinewave(
        freq=440.0,
        duration_milliseconds=500,
        volume=1.0):
    """
    The sine wave generated here is the standard beep.  If you want something
    more aggresive you could try a square or saw tooth waveform.   Though there
    are some rather complicated issues with making high quality square and
    sawtooth waves...
    """

    global audio  # using global variables isn't cool.

    num_samples = duration_milliseconds * (sample_rate / 1000.0)

    for x in range(int(num_samples)):
        audio.append(volume * math.sin(2 * math.pi * freq * (x / sample_rate)))

    return


def save_wav(file_name):
    # Open up a wav file
    wav_file = wave.open(file_name, "w")

    # wav params
    nchannels = 2

    sampwidth = 2

    # 44100 is the industry standard sample rate - CD quality.  If you need to
    # save on file size you can adjust it downwards. The stanard for low quality
    # is 8000 or 8kHz.
    nframes = len(audio)
    comptype = "NONE"
    compname = "not compressed"
    wav_file.setparams((nchannels, sampwidth, sample_rate, nframes, comptype, compname))

    # WAV files here are using short, 16 bit, signed integers for the
    # sample size.  So we multiply the floating point data we have by 32767, the
    # maximum value for a short integer.  NOTE: It is theortically possible to
    # use the floating point -1.0 to 1.0 data directly in a WAV file but not
    # obvious how to do that using the wave module in python.
    for sample in audio:
        wav_file.writeframes(struct.pack('h', int(sample * 32767.0)))

    wav_file.close()

    return


for note in TONE_FREQ.keys():
    append_sinewave(freq=TONE_FREQ[note], volume=0.01)
    # append_silence(90)
    save_wav("%s.wav" % note)
    audio = []
    print("after save %s" % note)