I have been trying to start learning Vietnamese as a beginner. There is no sound for eith the web version (Chrome) or the iOS app.
Is this a known technical issue? Please help.
There is audio on the Vietnamese course.
It is human recorded audio, rather than TTS, and because this needs more time and expense to cover every sentence, some sentences (hopefully a minority of them) are without audio.
There is however a current technical issue. Some users are experiencing problems with audio. Please see canspeakonlyone's post from about the same time as yours for a firsthand description: https://www.duolingo.com/comment/24456457
I am not confident enough to call it a "known" tehcnical issue, as I don't want to imply that Duolingo have noticed it - I haven't seen any staff post about it.