I’ve been working on adding audio to the Keybow 2040. I’ve not finished all the code but I’ve got the hardware and a demo working now. The addition is fairly straightforward, it just needs new 12mm spacers to replace the supplied 4mm ones and two passive components to make an RC filter to tame the PWM carrier in the audio.
More excitement:
And another video with a longer demo, this one shows how to make your quiet linear keys sound like an IBM Model F keyboard.
And a Keybow 2040 with added I2S audio amplifier (based on Analog Devices MAX98357A) and stereo speakers: Instructables: Macropad With Digital Stereo Audio Using Pimoroni Keybow 2040 and MAX98357A Audio Board (HID + MIDI Capable)
Very nice writeup. I use I2S together with MP3 in many projects, so this is certainly something to try. If you run one of the keypads with WLAN support (i.e. Pico-W/Pico2-W), you could even stream the MP3 from the internet.
Another thing to try: swap in a DF-Robot DF-Player Mini. This is only necessary if the MCU is busy with other things and you want to have full asynchronous playback.