Amplitude determines loudness or softness. Use the waveboard to demonstrate changes in amplitude. Amplitude can also be demonstrated by striking the tuning fork and moving the oscilloscope microphone closer and farther away. The waveboard only allows simple waveforms. Introduce the oscilloscope.
Oscilloscopes show sound waves.
Use the oscilloscope to demonstrate complicated waveforms. Talk about how waves can be mixed. Sometimes this increases the amplitude of the wave. Sometimes it distorts the waveform. Sometimes it cancels out parts of the wave.