Mass Loaded Vinyl (MLV) is a flexible and successful soundproofing content for windows, produced from PVC and vinyl having a hefty substance including barium sulphate or calcium carbonate. A simple way to seal these gaps is to fill them using an acoustic caulk, for example Environmentally friendly Glue Acoustical Caulk. https://fire-resistance66543.angelinsblog.com/37648357/the-ultimate-guide-to-acoustic-secondary-glazing