Well... dontcha kind of have that taken care of with ONE of them at least ;)
Actually, the females ARE bigger, and very obviously plumper. ESPECIALLY when they're full of eggies! They don't reach sexual maturity until around 9 months of age, though. . . which means nothing. Cuz' we have no idea how old ours are!
I wonder why a male in a tank with a female would even bother singing? But I am ALL EARS for that one! Have you googled it yet? There are sound clips, it's a sweet little buzzy sound, I'm sure it's easy to miss, but I'll be waiting!