Sounds like dealer BS to me. Motability rules allow customers to add optional extras to any new car order as long as they pay for them (most dealers will offer a discount on such extras). If an upgraded sound system is an extra cost option for the Tekna then you should be able to add it. Sounds to me as though it may suit the dealer for you to have the standard car – perhaps because they can get one quicker (cancelled order, maybe?). Might be worth a call to another Nissan dealer and ask what the lead time would be on a Tekna with the upgraded sound pack and see what response you get.