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703 is overkill for the inside of the wall. Rockwool or r19 is fine given your construction. The insulation won't contibute to blocking sound, it just keeps the wall cavity for resonating. You also need to use 2 different thicknesses of drywall in your layers. So if the ext.wall is doulble 5/8 the inside wall needs to be one sheet of 5/8 and one of 1/2.
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