Taking into account the information provided by this answer, in order to power an audio device with the following specifications:
is it correct to say that I could use a power supply providing an output tension between 9 and 20 volts? Furthermore, since no information about signal amplitude are provided, is whatever amperage sufficient to power the device?
Without the documentation from the manufacturer we can't know for sure. However here is an educated guess based on USB standards.
The amperage is promotional to the voltage. Meaning 20v is roughly 2x as much as 9v so if it draws 2A and 9v then it will take roughly 1A at 18v.
There is a formula, amps * volts = watts
so 2A* 9v = 18w
1A* 18v = 18w
If it has the ability to be USB powered it is very likely that the number of amps required is very low.
USB 2 is 5v at 500ma or 2.5w
USB 3 is 5v at 900ma or 4.5w.
There is a special charging mode where 2-2.4a of current may be delivered.
In this case 2.4*5 = 12w.
Now if it was USB 3.1 there is 100w mode, but your device sounds like it predates this standard.
Since 12w is the absolute highest a USB 3 port is allowed it can't draw significantly more than that. There is a loss of 20% (possibly higher) depending on the quality of the power supply conversion circuits.
9V at 3a should be more than enough to power this. Even 9v at 2a is 18w. As 9v * 1a =9w which is less than the theoretical 12w a USB could deliver so I wouldn't risk it.
The power supply you added in the comments is 1.2a at 15v =18w thus roughly 2A at 9v would be fine. Honestly, I would try and go a bit higher like 2.1 or 2.2 if that is an option.
Thus anything that totals 18-20w and it 9v or greater but less than 20v should be fine. So on the low end 9v*2 =18 on the high end 20v* 0.9A -18w.
