We could represent any classical music using human voice. Why?
It's true that most instruments that can produce identifiable pitched notes create sounds that are made up of partials following the harmonic series. The human Larynx can also create this kind of sound, so it has the basic 'ingredients' to sound like many musical instruments.
Furthermore, the resonant cavities of the mouth act as a very powerful filter, allowing that harmonic series to be shaped to sound more like the response of particular instruments.
The way the amplitude of the voice is breath controlled also allows the envelope of the sounds the voice makes to be varied in a very flexible way.
The tongue and teeth can also be used to shape noise from the air moving through the mouth into percussive sounds.
There are lots of limitations in the human voice too:
- The basic spectrum of sound created by the voicebox isn't an exact match for many instruments.
- There are limits in how low and high the voice can reach
- the mouth can't really make very percussive-sounding envelopes for pitched sounds - (it's hard for a voice to sound like a xylophone)
- there are limits to the volume of the sound that the human voice can produce
So while the statement "We could represent any classical music using human voice" isn't unfair, it would only be a representation, not an exact match.