Course Lacks Sound on Android?

Hello Duo!

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A little backstory: I live in a neighborhood where a lot of businesses are Vietnamese owned, and I wanted to learn more about the language because I like the people—they're friendly—but, there's definitely a language barrier as most proprietors only speak in the most atrocious broken English I've ever heard. However, I've had to give up on learning Vietnamese from Duolingo because it's a tonal language and the app is completely useless without any guiding audio. Apart from that, I have an interest in learning foreign languages. In fact, I've tried quite a few courses so far, just to wet my beak a little and see what works for me and which languages I prefer best. However, a couple of the courses I've tried lack the audio supplementation to make them useful. The Vietnamese course for example, almost completely lacks audio. It wasn't the only one, but it's the only one that comes to mind at the moment.

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FYI, I downloaded the Vietnamese course again prior to posting, just to make sure that the issue was still present. I was able to hear a single audio playback, which is why I used the qualifiers "almost completely" instead of just "completely" to express how much of the course lacks audio. Since, the course previously lacked 100% of any audio integration as far as I could tell (I did not progress much without audio before abandoning the course).

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So, does anyone know what's going on here? Does my phone not support Vietnamese audio for some reason? Is the course incomplete? Is this happening for anyone else?

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^(If this is a repost, please excuse me as I searched and couldn't find anything similar.)