I assume these are conversions? I doubt it will fail any sound tests because of a chimney airbrick.
Air bricks in an unused chimney are a must to keep a good airflow in the chimney itself and prevent damp from forming in there....
As a priority, I would ensure that the flats have good ventilation, especially in bathrooms and kitchens.
Without stereotyping, tenants in flats seem always seem to have more damp condensation problems than anywhere else. This is because the flats are small, usually do not have enough ventilation and tenants usually do all the wrong things which encourage damp/condensation. The big one is that clothes are dried indoors on radiators (even if there is a vented tumble dryer there) as a way to keep energy bills down.
So as part of your development, I would ensure that bathrooms have good automatic extractor fans with a humidistat. The kitchen has an automatic extractor fan with humidistat or at the least a good oven extractor hood which is vented outside (tenants do not usually use the cooker hoods, so that is why an automatic fan is useful there too).
If you highly insulate a flat, that is great, but if you do not think about ventilation and airflow, then prepare to get damp and mould problems in the future.
