Greetings D —
First, create some Sounds with EventValues, and establish bi-directional communication with those EventValues and Processing. Then, in the Tool, create a variable and use Set Value Options... to associate the value of that variable with one of the EventValues. They can be writeOnly or readAndWrite. Each time you change the value of that variable in the Tool, it also changes the value of the EventValue in the Sound. And since that EventValue is communicating with Processing via OSC, the change in value is also transmitted to Processing.
When you run the Tool, use the File menu to open the Virtual Control Surface (the VCS should be open so that OSC messages can be sent).