To the untrained ear, Dutch and Flemish voices sound the same, but professional producers can hear the difference immediately. Flemish uses shorter vowels, softer consonants and a more relaxed intonation. Word choice and expressions also differ: a Flemish listener can tell straight away whether a advert was recorded in the Netherlands or in Flanders.
For the Belgian market, a Flemish voice-over is the right choice; for the Dutch market or for a neutral European-Dutch product, producers usually opt for a Dutch voice. Voices books both profiles separately and, if in doubt, helps with a short casting session.