An oscilloscope draws voltage against time. Before the microphone there was the oscillograph, which photographed a trace on a moving film drum — and before that, the galvanometer, a needle that swung and wrote a line on a moving strip of smoked glass. Every one of them is a way of turning time into a picture.
Swap the waveform and the instrument does not care. The trace is amplitude against time, which means any quantity you can turn into a voltage can be looked at: nerve impulses, the tremor of a bridge, the current in a speaker, the beat of a valve.
The amber trace is the second channel. Real scopes show two so you can compare — the reference signal against the thing under test, and the shape of the difference is where the information lives.