I had the issue of Visual Studio not recognizing methods from the Wwise API, when I wrote them in code.
I found out the issue.
The project I was working on utilizes Assembly Definitions, which are a way in Unity to control when specific scripts should be compiled. I had to go to the Assembly Definition asset in the script folder, and add the different Wwise API modules (or namespaces, not sure what to call them) to a list within the Assembly asset, before the Wwise API could be recognized within scripts that adhere to this Assembly.
Hope this solution was useful!