If anything, a bunch of cockatiels or budgies would sound more like you're out in the bush area of Australia, or something.
Parrotlets are native to Ecuador though, which I'm pretty sure is on the more tropical side. So people wouldn't be too far off to say Jan's house sound like a rainforest.
So I found this video:
In addition to the pearling, the owner said she had been DNA tested and even laid eggs.
So I guess while whistling in female cockatiels isn't necessarily common, it's not impossible for them to have a clear whistling voice.