If smells are drifting through, it is probably where joists have been pocketed into the brickwork, and where slovenly bricklaying has left gaps that were not hidden by plaster. There will be gaps under the floorboards that are out of sight.
It was normal practice for the best quality bricks, and the most skilled bricklayers, to be used on the front of the house, for show. The back of the house would be done more cheaply, and the internal walls, which were going to be concealed by plaster, just had to be good enough not to fall down on their own.
You can fill in the gappy brickwork, a soft mortar mix will do it (clean out dirt, dust and loose material first) or you can use expanding foam (after cleaning, spray the hole with water, to make the foam stick and expand). Use a foam gun. I use pink fire foam, which will char but not burn, and blocks smoke and flame. Probably you will never have a fire, but, why not?
The foam expands more than you think, and is very sticky, and very difficult to clean off floors, clothes, shoes, skin, hair and eyeballs. Cover and tape everything. Newspaper and cling film will do. Have a large supply of disposable gloves and change them immediately they get sticky. Spray water on drips and bulges. It makes the foam skin over.
Once hardened, you can cut away excess with a serrated knife, followed by coarse sandpaper. Your fingernails will eventually grow out.