The problem apparently stems from the fact that these applications are only using the handset's top microphone, which should only be used for noise cancellation. Apparently, this problem has also been discovered in older models, including OnePlus 5, OnePlus 5T and OnePlus 3. Not all applications are affected; Instagram, for example, does not seem to have this audio problem when playing sounds recorded by a OnePlus phone.
Some OnePlus 6 and OnePlus 6T users wonder if the microphone issue is why they have trouble activating Google Assistant when they click "Hey Google." Fixing this issue may require nothing more than a software update, even if there are third-party applications involved.