Hey @sil, I’ve been using Duolingo for about 4 years to learn some french.
I have been putting in near the minimum amount of effort so I’m certainly not fluent, and I haven’t had much chance to practice using it yet, but I can understand written french pretty well now. Today I have a streak of 1365 consecutive days (3.5+ years) which I’m quite motivated not to let die (I’m also helped by another automatic system that would charge me if I didn’t practice).
I’ve always had the speaking exercises turned off because I didn’t believe in the accuracy of their automatic rating - this was a big mistake, because now I am really really bad at speaking any of the french that I know. According to a language tutor I know it actually doesn’t matter as much about the accuracy of your speaking as much as it matters to practice speaking out loud at all.
The audio clips you mentioned are prerecorded, but using text to speech engines rather than real people.
To give a bit of background on the money situation, you don’t have to pay at any point, ie to get to any level, so don’t worry about that.
In terms of their business model I think they originally just burned through investor cash with a plan to make money through human-created paid translations like @basje said. As a result they never used to have ads or any kind of paid options. But since abandoning that they have in the last year or so added ads, a paid “pro” option that gets rid of ads and enables offline lessons, and those one-off payments that keep your “streak” alive even if you miss a lesson.
There’s a lot of debate about how effective Duolingo is for learning a language as an adult. As you say, it doesn’t bother teaching you much in the way of language “rules”, you learn in a more natural way (just like you didn’t need to explicitly learn any rules to speak your native language). However I think most people agree that it will take you a much longer time to learn in this way as an adult if you don’t learn some of the rules alongside it, because we lose much of our natural ability to “absorb” language as we get older.