I'd probably wait for eggs at this point. From the 5 month age, I'm betting laying will start any day now, especially with her going to the nest box. But yea, eggs are a surefire way to prove female. A male will never be able to lay. Lack of eggs is a bit of a fuzzy area though.
When you do...
A video may be helpful on this. Hens can be generally very vocal during certain events of the day. I'm curious if this is some variant of the egg song, some distress call or just general hen chatter. My girls get loud for any number of reasons - heard a noise, saw a shadow, or even someone else...