Sounds to try:

• Ssssshhh
• Clapping quitely and loudly – what differences do you see in the oscilloscope?
• Whistling, sustained and at low and high pitches – same question, what differences do you see in the oscilloscope?
• Singing long vowell sounds – even single "notes" contain sound energy at multiple, regularly spaced frequencies, which are all added together in an oscilloscope, but separated in frequency spectrum analysis tools like these under the mic icons here and here.
• Beatbox!

This simple but fun demo makes use of p5.js and Tone.js.