The 30-year-old Frenchman played against Barnsley in the Carling Cup and his only league appearance last season was in the 1-0 home defeat to Bolton, Kevin Keegan's last game in charge.
Sommeil has been hit by a number of injury problems but, when fit, had slipped down the pecking order behind the likes of rising stars Nedum Onuoha and Stephen Jordan.
Sommeil, who has failed to impress at either centre or full-back for City, was signed by Keegan from Bordeaux for £3.5m in January 2003 and made 14 appearances as City said farewell to Maine Road.
He played a further 18 games the next season before being loaned to Marseille, with Daniel van Buyten moving the other way on a similar arrangement.