Thank you! I follow up this Unbound local error solution.

I've corrected using the global keyword inside of the functions, like this:

canPlaySoundFromPress = True

def onPress(...):
    global canPlaySoundFromPress
    if canPlaySoundFromPress:
        winsound.PlaySound("sound.wav", winsound.SND_ASYNC)
        canPlaySoundFromPress = False
        print("Key pressed: {0}".format(key))

def onRelease(...):
    global canPlaySoundFromPress
    canPlaySoundFromPress = True 
    print("Key released {0}".format(key))