I love projects involving our senses! Sight, sound, touch, smell, taste. Adafruit's inclusion of a microphone and speaker on the Circuit Playground board provides a gazillion ways to explore our sound sense.
Let's review what sound is and build our skills from there.
You can think of most phenomena around us as existing in the form of waves. From the lowest frequency waves of an earthquake to high frequency cosmic rays, the term you will hear is that waves go "from DC to Daylight". That refers to the broad spectrum of wave action in the energy around us. Below are some waves of differing frequencies (the top one is at a lower frequency, the bottom the highest frequency).
via Public Domain, Wikipedia
There are so many frequencies! What we call sound is air vibrating at frequencies our ears can perceive. For a child, that would be 20 Hertz (cycles per second) to 20,000 Hertz (20 kiloHertz). For grandparents, they might not hear that upper limit, maybe 15 to 17 kiloHertz. Frequencies above 20 kilohertz may be audible by animals (that is where dog whistles fall, in the range 23 to 54 kiloHertz).
Sound can be soft and low like a thud or high and loud like an opera singer on a high note. How do we produce so many sounds? We can define how we characterize the sounds we hear:
Frequency: how fast the sound wave vibrates back and forth. One frequency would be monophonic, playing several sounds of different frequencies together gives polyphonic sound.
Duration: How long and short a sound is made.
Loudness: How strong the sound is (in the waves above, how tall the wave is shows the loudness or intensity).
Timbre: how a sound sounds, say dull like a plucked string vs. a "pure" musical note.
Spacial location: where your ear perceives a sound coming from. Think of a train passing you, the location changes from your perspective. If you note a sound change to your ear, this is due to more science in the form of the Doppler Effect.
Ok, enough talk, let's make some sounds!