In my girlfriend's house is a windows connecting two rooms. We don't want this windows anymore, on the one side is a wardrobe placed in front anyway.

My plan is to leave the window where it is, and simply fill the offset to the wall (50mm) with maybe some kind of mineral wool, screw a sheet of chipboard, OSB or Multiplex over it and use it as a kind of pinboard.

Two questions:

Which easily available material (preferably at Global House) is the best material for sound proofing? Should come in 50mm thickness.

Does Global House (or another shop) offer cutting a sheet of chipboard or OSB or Multiplex to specs? I need about 150x100cm.

Sound reduction needs mass so that structure will do little.
 

Multiple layers of MDF will do a much better job than anything else that’s easily available, you also need to seal the edges of each layer to its surround.

 

Further neither MDF nor OSB are good pinboard material, however I haven’t made a pinboard so while I can tell you of material that doesn’t work well I can’t tell you which ones don’t.

Mineral wool & fiberglass both have good sound reduction qualities are are routinely used in commercially produced sound attenuating products. 

 

As far as I know, cork is the preferred material for pin-board. Why not just buy a ready-made pin-board, white-board or combination board from OfficeMate large enough to cover the hole?