How do I create a volume slider?

Unity
1

I have sounds in two different banks, how do I create a slider for each bank?

2

Banks contain the data for the audio, you will want to control the volumes of Buses
https://www.fmod.com/resources/documentation-studio?version=1.10&page=fmod-studio-concepts.html#routing-and-mixing

There are a number of tutorials available on YouTube, for example: