On the website it sounds like /la sɛnə/ (the last sound is a mistake, it should sound like /la sɛn/). I have had problems with the fast « la » sounding like /læ/ (like "lad" without the "d") in other sentences but this sentence sounds fine in that regard.
You should report any errors you come across, but be aware that audio errors are very hard for the Duo team to fix at the moment so we will have to put up with them for a while.
I don't work on their systems but I am a programmer. I imagine that they are using third-party TTS (text-to-speech) software. It probably operates as a black box, so they put their words in and a recording comes out. There are probably very few options for them to fine-tune each recording. As the users we tend to focus on the relatively few errors that it makes, when the software is actually amazingly good most of the time. However, when an error can't be corrected it is good for us as students to be aware of that so we don't incorporate that into our own usage.
As an example of how tricky the software can be, the word "pin" is pronounced /pɪn/ phonemically (potentially many different pronunciations heard as the same sounds) but [pʰɪn] phonetically (exact sounds). The word "spin" is /spɪn/ phonemically and [spɪn] phonetically. [p] (non-aspirated) and [pʰ] (aspirated) are allophones of /p/ in English, although in other languages they could be separate phones (such as a lot of Indic and southern Asian languages). The superscript "h" can be heard as a slight puff of air just after the "p". If the software gets this wrong, it will sound unnatural, but you mightn't be able to explain why.
It sounds like you've worked in, or at least studied, natural language processing. I've studied predictive analytics but I haven't touched language processing for the reasons you just cited. :-) I never considered that Duo might be using a black-box TTS tool, but you're probably right. The best ones available for commercial use aren't cheap and, since Duo is free, I guess we can't complain too much.