Vietnamese Audio for everything
Having the option to hear audio for every problem would help tremendously since it's so difficult for English speakers to pronounce.
The course contributors have stated that a limited budget meant that not everything could have audio. If a particular word is tripping you up, I recommend forvo.com, where native speakers of many languages (including Vietnamese) upload pronunciations. You could also give google translate a go, though I'm not sure of the quality.
It's a program that is free for us so it's not like we are paying the contributors anything. There are a lot of people who like this program the way it is... free, and very educational. This Vietnamese learning track for english speakers is new and I appreciate that we have it now. The other language sites/apps where you have to pay like Busuu, Babbel don't even have Vietnamese and the price of Pimsleur and RosettaStone is ridiculous in my opinion. I use forvo and find it is a pretty nice tool, and very helpful.
Syntaxosaurus is correct - adding "real audio" is very expensive. It's also very time-consuming. These are the two main reasons that the "real audio" courses (Vietnamese, Irish, Ukrainian and Esperanto) can't have audio on every word, only on the sentences (and even then, some sentences miss out).
Duolingo would strongly prefer the courses to have computerised TTS speech, as it is a) far less of a drain on resources and b) you can have audio on every word.
Unfortunately, for the language like Vietnamese, there was either no TTS available, or no suitable quality TTS.
Doesn't have anything to do with being in beta - it won't change when they graduate. What "may" help is if a language can get up to the level of creating a Tree 2.0 in future, which may have more audio added (I think the Esperanto guys said they were).
I contacted many big companies specialized in TTS or auto-generated voice message. None offers Vietnamese TTS. The closest one you can get is a STT (speech-to-text). Unless Duolingo can get their hand on Vietnamese TTS in a near future, this course should not be expected to be fully covered with audio. Sadly.