He made 28 appearances for the Saddlers and scored four goals but Colin Lee decided not to renew his contract this summer.
Forest manager Paul Hart has made no secret of the fact he needs more depth in midfield after persuing Leicester's Matthew Jones, Manchester United's Michael Stewart and former Stoke midfielder Bryn Gunnarsson.
Sonner trained with the Forest squad on Friday and is expected to play some part in their pre-season friendly against Scunthorpe United at Glanford Park on Saturday 26 July. (KO 3pm)