The Metro M4 can play PCM 16-bit Mono Wave files at 22KHz sample rate. Thankfully you can find some files that were made just for this project that will work.
You can download the following sounds and place them into a new folder called sounds in the CIRCUITPY drive.
If you would like to make your own sounds, the following link will take you to a guide that can help you format the WAV files. Keep in mind that you will want to make your files small, as WAV files can fill up your storage space quickly.