What is the procedure to notify the use of microphone frequencies at a concert? Do specific frequencies have to be reserved or only a number of frequencies in the bands concerned?

A frequency band is assigned. The list of frequencies allowed or not allowed in the region in question, is available here: http://www.bipt.be/en/consumers/radio/micro

The frequency bands for which no licence is required, insofar as they are used with a certain power, are also included in that list.