Were it the DC voltage/current, from an amp's power supply, modulated by the amp's output devices, into an amplified musical signal; it would appear much more complex, but: still a sinusoidal wave
The musical signal is more than a sinusoidal wave, otherwise it would not be considered musical! We talk of the harmonics that make say a flute sound like a flute, or an electric guitar sound like the distorting amplifier it is connect to.
Fourier theory has it that any repeating waveform or musical note can be represented as an infinite sum of sinusoidal waves, being the base frequency plus all the possible harmonics or overtones. You can create a graph of the frequency spectrum of the note, although the original note exists entirely in the time domain.
You can apply a mathematical Fourier transform to convert the time domain into the frequency domain, and back again (but not perfectly).
This idea is so pervasive that many audiophiles speak and think in the frequency domain - the treble does this, the midrange does that, and the bass something else.
The only thing I can think of in nature that converts the time domain to the frequency domain is our ear / brain system, which fires complex patterns of neuron activity where the original neurons which fire respond to particular frequencies, but fire at rates depending on loudness. The initial firing pattern depends on arrival patterns in time.
There are a couple of issues. Obviously we do not hear high frequency harmonics above say 20-kHz. To reconstruct sharp transients (for example square waves) very high frequency harmonics are needed to capture the leading edge. And mathematically without these high frequencies the Fourier transform wobbles before and after the leading edge. As more higher frequencies are added, a spike appears extending the leading edge. Ouch