There are other Anabantoid fish that make croaking sounds. Trichopsis vittata is sold as the croaking gourami. I'd never stumbled across any reference to a Betta splendens, or any Betta doing this, but I went digging with this excellent question. It seems they may be able to make a sound people have interpreted as coming from the labyrinth organ, but those are hobbyist reports and they haven't yet made sounds it could be proven for certain came from the Betta. I don't think a lot of research has been done though.
I found this:
Following recordings carried in
our lab (personal observations), B. splendens does not seem to produce vocalizations and
lacks sonic organs (Ladich & Popper, 2001).
Thanks to your question, while drinking my morning coffee, I read about how Bettas hear. Males have better hearing than females, but need more oxygen as well. That's new to me.
Their inner ear has evolved into a breathing (labyrinth) organ for surface air, so their hearing at low frequencies may be connected. There have been low pulsing fish noises recorded in their marshes, but researchers thought they came from gouramis, many of which use noises.
The labyrinth is complex and the name of the game for Betta breeders is to cultivate mutations. Sometimes changes have effects elsewhere, so maybe some Bettas have sound making abilities. I guess...maybe? Keep listening!
