The only information I was able to find on frogs making bubbles is from a German web site.
Translated: "On the 14th day of their lives you can watch the little swimmers taking in air at the surface and then making bubbles under water, exhaling just like adult frogs."
So I would assume that your frogs are merely exhaling.
These frogs, like their larger cousins, the ACFs, are quite vocal during breeding season. Unlike the ACFs, only male ADFs call.