Speaker Amplifier circuit problem - need help

[Steve8-44]

I have to design a speaker amplifier circuit for my car (E30-SEDAN1994). We are told the impedance of the speakers and the minimum voltage they require and also the voltage source. After researching I have found that the LM386N-4 is a popular choice. According to a datasheet of LM386N-4. I've seen many circuit diagrams but what I don't understand is how to pick the voltage gain needed for the amplifier.

I originally thought that you would need to know the input audio signal voltage and when you multiplied that by the gain that would need to give the minimum voltage required by the speaker, but it appears that what matters is the current which I don't get.

And are you able to design such a circuit without knowing about the audio input signal? If you do need to know about it, what is it you need to know about it and why?

I would greatly appreciate any help as I am very lost.

Thanks in advance.