How to make the volume of hotel fire broadcast infinitely adjustable?

Just install a simple power amplifier, which is the kind used in small speakers now.

But now there seems to be a switch that sells stepless adjustment. You can ask where there are audio appliances or lighting switches.

Or add a stepless frequency converter.

If you want to do it again by yourself, ...

It is regulated by a bidirectional thyristor, just like a dimmable desk lamp. Its characteristics are low cost, small size and light weight. The disadvantage is that the voltage is adjusted by the conduction angle of the thyristor, and the output is not sinusoidal current, which will produce a lot of harmonics and cause great interference to the nearby radio and stereo.

The other is to use different taps of the iron core inductance, that is, to change the inductance of the inductance connected in series with the speaker circuit to change the voltage and adjust it. The advantage is that the reactance is used to divide the voltage, and there is no harmonic generation and no interference to other electrical appliances. The disadvantage is that the inductor with iron core is bulky and consumes some electricity.