Lath and plaster can be a challenge, but yes, you can indeed put in-wall and in-ceiling speakers in old homes. However, you do have to do some more preparations. Here are some basic problems that do come up with this type of install:
Sometimes there’s a layer of drywall applied over the top of the original lath and plaster wall. In this case, you have to extend the screw length of the in-wall or the in-ceiling speaker as your wall thickness is going to be over 1 1/4″. You can get longer screws at your local hardware store.
If you are installing an in-wall speaker, use the template to score the plaster with a utility knife before you cut the hole. You will need plenty of extra blades because the plaster will dull them very quickly. Then you can cut the lath behind the plaster. Try to keep the vibration down to a minimum. Try not to use power jig or cutting tools. Hole saws are fine.
Before installing, note that it’s a good investment to buy in-ceiling speakers that are of the same size. Then you can get a hole cutting saw that matches your speaker size. With a hole-saw, the openings will be consistent in size, and with that tool, you’ll be less likely to crack the plaster on the ceiling.