Male and Female Voices
I know that in French class when we have a substitute and he's a dude. I can't understand half the things he is trying to say, because the tone of his voice is so much more diff. than my normal teacher. So I think we should have male and female voices so in real world interactions you can understand BOTH genders.
I doubt it's just because his voice is deeper, though. Maybe he has a different accent from your female teacher? They might have learned French in different parts of France (or Switzerland, Luxembourg, Quebec).
Male or female, it's still a robot voice and will always sound different from the endlessly variable ways that actual humans speak the language. We should probably supplement our Duolingo study by listening to audio of real speakers in movies or on Youtube or http://www.bbc.co.uk/languages/ or other language learning resources.
As a native speaker of french, i'll have to tell you this is likely to be the main reason. In my view the speech of a French Canadian and that of a person who grew up in France are about as distant as in English, say, US and Scottish accent: each group undestands without much trouble most of the speakers, but there's a "getting used to" factor that makes it seamless only after some exposure. I am sure you'll get better at understanding your sub after a couple of sessions, just the same way... There is some vocabulary that differs (about like between US and UK english), but i guess you're going over class material anyhow so that shouldn't matter much for you.
On your idea of using several voices, i'm unsure: it sounds like a great idea on paper, but depending on the language and if you want to encompass other factors than just gender (age - a 3 year old is insanely difficult to understand the first time you hear one in a language you otherwise are fluent in-, accents, "level of speech" - BBC news anchor or football-watching in a pub, ...) it can get pretty much endless.
If it was doable (the argument of a robotic voice being a robotic voice seems very convincing to me), it'd be a great optional feature. Go to the options, set "female teenager" for a couple of weeks and so on. I don't think I'd like the voices to change in the default course of things though: too much to cover and distracting against other tasks.
In my experience of learning English, then spending some years in an English speaking country, I think what helps smoothening the shock of many voices-many accents is listening to material. It is really easy with English, what with all the movies and TV shows, but it is possible in other languages as well. Get DVD's of foreign language shows or movies and watch with original language subtitles. Great way to learn idiomatic expressions, too! It's tedious at the begining, after a while it feels not much different than just lying on the couch and get good distraction. Stay away from animation stuff and anything dubbed though - you need to see the lips moving, it's so much more helpful than one'd think.
On the In A Month Series (it's in the android and iTunes stores) they use a method where two native speakers say the sentence (a girl first, then a guy) so that you can hear the slight differences when males and females speak. Of course, I would get bored of hearing the same thing twice and end up skipping the male's speaking part. They would sound a bit different at times.
This is the website. They have demos (if you have a tablet or smart phone) if you want to see what I am talking about. http://www.learnlikekids.com/