Hello Bruno, the holes are sort of a pressure relief. When you blow into the melodica you can hear the sound change from a quick attack to a soft relief. All (5) holes on my clavietta were open to the back side. Not knowing the actual theory on these holes I can only tell you what I have found in closing them. When I closed the holes I no longer had a leak. After changing the washers, gasket in the air chamber and sealing the bolt holes on the air chamber. Another theory is that these holes may be unknown factory adjustments for the clavietta?