"They see a cat."
Translation:Ils voient un chat.
I'm not sure what you're saying "every single time" about, but ils voient is the correct conjugation for voir here. Ils voyent is an archaic form, like "thou seest", except "they" instead of "thou".
Duolingo doesn't display the "play audio" button until at least one comment is posted. So I wrote that partly because I wanted to hear the pronunciation and also because I consistently write "voyent" when it should, as you correctly state, be "voient".
But I also suspect that's a fairly common error. So I also wrote what I wrote because I'm sure someone will see this and relate to my reaction and it could hopefully serve as a reminder to both them and I in the future.
Ils voyent was correct in Middle French, during the 17th century and earlier :-)
I haven't ever had a problem with that/those words, but the number of times I used to write manteau when I meant chapeau and vice versa was just embarrassing...
There are quite a few examples of words that sound identical but have different meanings, just as there are in English. Often, the only way to tell them apart is the context. So if you're talking to someone about a group of people and you hear "Il voit", there's a good chance they're actually saying "Ils voient".