Go to AkAudio.Build.cs and find where it says:
PublicLibraryPaths.Add("$(DXSDK_DIR)" + Path.DirectorySeparatorChar + "Lib" + Path.DirectorySeparatorChar + LibFolder);
and replace that with:
string DXSDK_DIR = Path.GetFullPath(Path.Combine(EngineDirectory, "Source/ThirdParty/Windows/DirectX"));
PublicLibraryPaths.Add(DXSDK_DIR + Path.DirectorySeparatorChar + "Lib" + Path.DirectorySeparatorChar + LibFolder);