I saw a reference in an article to a small simple cheap mp3 shield for Arduino and it seemed like a fun and interesting idea. The shield incorporates an SD card holder, and in fact is only slightly larger than a micro SD card in a 16 pin DIP format. I found that I could buy a couple on ebay for £5.
A quick look on the internet and I found a couple of useful tutorials at buildcircuit.com. It turns out you don't even need an Arduino, you can wire up the mp3 player to 3V3 power and a speaker, put some music on an SD card and start playing. There is a small catch - the player doesn't play MP3 or WAV files only an unusual format called AD4 so you have to convert them first. I used some sample files which were available and copied them to the card as 0000.AD4, 0001.AD4.... etc. Once the card was inserted I simulated play by grounding pin 9 and a snippet of music played. It was only about a second but it was a great test to get started with.
Next came the Arduino setup. The tutorial suggests an ESP8266 but any Arduino will work, you just need 4 digital pins, and I used a nano. There is a simple WTV020 library available in the Arduino IDE Library Manager which I installed. It only consists of a single C++ program and you can see what functions are available. I used an article from electronoobs to help me a bit with the setup. The sample sketch shown in the article is also provided in the Arduino library so you can simply use this sketch, amending pin numbers as necessary. I compiled and downloaded the sketch to the Nano and it played a little music.
I found a text controlled by keyboard input via the serial monitor much easier to experiment and used a little sketch to test the functions.
The results were excellent. The music played is very reasonable qulaity and it is easy to change track, stop, pause etc.
Now I needed to use my own sounds/music. AD4 format isn't well known or supported. There is a utility SOMO or UsbRecorder I downloaded from 4dsystems.com.au. I first took an mp3 track and converted it in audacity (mix down to mono, Normalise to -6db, export Quality=16kbps). I then used SOMO to convert it to AD4. Converting the mp3 track to low quality mono decreases its size to about 5% but converting to AD4 increases its size about 5x. The newly converted track, once copied to SD card works perfectly on the music player and sounds good.
Since we have a multitude of ways of playing high quality music files it isn't likely that in practice we will have a use for a low quality arduino controller music player. However it will be much more useful for spoken words, we can get the arduino to speak! The player has the ability to play up to 512 files (0000.AD4 to 0511.AD4) so we have a vocabulary of 512 words to play with. Unsurprisingly I couldn't find a set of AD4 format words and even MP3 format words aren't that easy to come by. I found shtooka.net which has a large collection of about 4000 English words in a female voice "Mary" I could use.
