Drain clams, reserving 1/2 cup clam juice. Set clams aside.
Place pasta sauce mix in a small saucepan. Gradually add clam juice and milk, stirring with a whisk until blended. Bring to a boil, and cook 1 minute or until slightly thickened, stirring constantly with whisk. Stir in mushrooms and clams; cook until thoroughly heated, stirring constantly. Remove from heat; stir in sour cream.
Combine sauce and fettuccine in a bowl; toss well. Sprinkle with parsley. Serve immediately.