Most Chinese plywood products I've seen are terrible. I don't know about brownwood plywood. If I were you, I'd stay away from plywood unless I knew its quality or unless I could afford to scrap the cabinet if it started buzzing. Sometimes debris breaks loose in ply months or years after construction, causing a buzzing sound long after the speakers were first made.Most retail stores here don't carry Baltic Birch, so a casual purchaser would say it's hard to get here too. But wholesalers stock it. You just have to know someone in the business of cabinetmaking. They can probably get Baltic Birch for you and I think it's worth it.
As for using a thicker material, that's fine. A slight change isn't going to make much difference but it is also easy to add 1/4" to each dimension to make up for 1/8" thicker wood.