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5V USB is now standard and very convenient.
But I see a lot of synths and audio projects working with higher voltages ... from 9V to 12V to 22V etc.
Are there physics reasons for this? In that components can only do their thing with higher voltages? Or could the standard analog synth components making up oscillators, filters, envelope shapers, delays etc be "scaled down" to work within the standard 5V / 0.5A of USB?
It depends on the USB spec, with 2.5W there is a lot of power to work with if you used low power opamps. There are many opamps that have quiescent currents in the uA range, the rest of the power is determined by the circuit (or filters and oscillators in this case) and also the power rails. Rails can go very low (lower than 1V). However, keep in mind that the lower you make the rails, the lower the dynamic range and worse SNR the circuit will have (all things being the same but the rails).
It really depends on how many opamps and what kind of audio quality (noise) requirements. Also low power/low noise opamps are usually more pricey than regular ones, so there are many tradeoffs that would be needed to make a design like this, but it could be done.
Classic analog synths (like the modern Doepfer A100) tends to be somewhat power hungry, because they are based on discrete BJT circuits (we are talking about 1960-70 technology here). So +-12 or even +-15 supplies are the norm (also for noise issues: more signal voltage is less susceptible to noise!).
Using BTJs also means high biasing currents (also for noise, and most of the BJTs are working in class A), so, keeping the A100 example the standard power supply has 2A on the +12, 1.2A on the -12 and 4A on the 5V (no idea of why all this logic power is needed).
So a full fledged analog synth is quite difficult to run on the +5V 500mA nominal USB supply (about 2A if you use a USB charges). So unless you dig into USB-C power delivery (which is way more complicated that two wires in the connector) you have some serious power limitations.
The dual 12V supply is easy to obtain with a couple of power converters. If you only need a couple of oscillators and filters it will probably handle the load.
Or you could simply do a fake analog synth with a DSP like most people these day do anyway
