Let’s start with saying that an audio signal only needs to carry information about the amplitude (aka volume) in a certain moment of time, you don’t need any other information like pitch for example.
One way is to have the radio signal simply get higher amplitude to represent louder sounds, and so they did, the louder the sound the stronger the signal, this is AM. However it sounds pretty bad because interference causes issues with noise
Another way is to have the signal get higher frequency rather than amplitude when it’s louder, so you are Modulating the Frequency, FM. This is much better because noise typically only affects the amplitude, not frequency, so it sounds much higher fidelity