I like to think of "nói chung" as "generally speaking". As in, a lot of people would say (nói) it is so.
I checked the audio on Google Translate and here. I can clearly hear both speakers pronouncing "nói chung" just fine, despite the different accents. It could be a hardware issue. Then again, Vietnamese is a tough language to learn so we need lots of practice to differentiate tones.
Follow this discussion to practice listening. Whist listening, close your eyes to focus your senses on listening. If I can get it, anyone can. There have been over a dozen sentences I have had to do much listening practice and am down to one.