WARNING: This page is about a different car, the 2004 Volvo S40, 2003 Volvo S40, 2002 Volvo S40, 2001 Volvo S40, and 2000 Volvo S40. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Identifying Seat Belt (Inertia) Reel
Courtesy of VOLVO CARS OF NORTH AMERICA.
Check that the seat belt (inertia) reel locks as it should. Test the function by pulling it out and jerking it repeatedly.