Wij is pronounced with a diphthong as in why or as in way. (Of course the consonant is pronounced differently.) We is always pronounced with neutral vowel (schwa), similar to the beginning of way.
Now here's the problem: The Dutch voice goes out of its way to speak a bit sloppily so it sounds natural. This isn't a perfect process. The second half of the diphthong gets cut off, leaving only the first half. Unfortunately, the automatic voice uses the way-like pronunciation, so the result is not distinguishable from we.
Of course this is precisely how the word we first arose, but now that the two words exist, Dutch speakers (have to) pronounce wij more carefully.
This is probably not going to be fixed soon since worse problems have persisted for a long time with some of the other language voices. The only workaround is to use the slow voice whenever something sounds like we but could also be wij.
PS: There is now a new voice, which is much better.
The problem with that workaround is that it would be a lot of effort that would have to be undone as soon as the Dutch voice is fixed. No doubt this will happen sooner or later. At the moment the constraints are: It must be technically possible to include the voice into Duolingo's API and Duolingo must be able to afford the licensing. I am sure Duolingo has chosen the best voice satisfying these constraints, but I am sure a better voice will be available sooner or later. (The developers might also find a technical workaround that forces the voice to speak correctly.)
We and wij are interchangeable. However, "wij" is sometimes used to emphasize that WE have something that you don't. As a result, using "wij" in normal conversation sounds a bit stiff, so unless you are emphasizing, use "we".
The exact same thing goes for jij and je when it is the subject of the sentence. However, matters become more complicated as "je" is also used as a replacement of "jou" (example: Hoe gaat het met jou?/Hoe gaat het met je?). In these cases, "jij" is not possible ('Hoe gaat het met jij?' is wrong!).
"Je" can also be a non-emphasizing replacement of "jouw" (2nd person possesive form). Your brother/Jouw broer/Je broer. Using "jouw" is emphasizing that it is YOUR brother, using "je" is a reduced, 'sloppy', non-emphasized form. As with "we", "je" is more often used as a subject but also for the possesive form.
Extra info on some exemptions: However, "je" cannot ALWAYS replace "jij" as the subject.
example: Are you and your brother coming too?/Komen jij en je broer ook?/Komen jij en jouw broer ook. Wrong: komen je en je/jouw broer ook?
The first translation is more often used than the second. "Je" cannot be used instead of "jij" here because the subject of the sentence is "jij en je broer" (so, plural, not 2nd person).
Yo escucho "wij" en este audio. Wij = w ay, we = w eh.
Here is a clear pronounciation at 00:02 and 00:17: https://soundcloud.com/carola-janssen/we-en-wij-niet-door-elkaar