You left your theremin at home and are lacking a suitable replacement for tonight’s big party.
Place two or more cell phones in close proximity to each other, make calls to each other (works best if the phones have a speakerphone mode).
Lots of distorted feedback and odd sound effects. Digital cellular phones compress and distort audio (there is also a noticeable transmission delay). Notice what happens when one handset picks up a sudden noise. It’ll emerge, slightly garbled from the other handsets a fraction of a second later, and this will continue for several more cycles until the original sound is unrecognizable.
For extra fun… try this with three or more phones. Also try with pairs of phones using different combinations of cellular carriers. In the USA, there are several different cellular systems, each of which handle audio slightly differently (different transmission delay characteristics, different compression algorithms).