I'm writing this before the OnePlus 5 was even announced. However, OnePlus already revealed some information regarding the audio recording quality of the OP5. The new OnePlus 5 will feature an improved microphone with higher dynamic range. They have posted a OnePlus 3 vs OnePlus 5 audio quality comparison video on their official YouTube channel showcasing the difference.
I have to admit that I wasn't expecting such a huge different. I don't own the OnePlus 3 but many people already stated in the comment section that the OP3 audio recording is really that bad. Some didn't believe that the OP3 mic records audio so badly until some people proved them wrong with their own videos.
There is no reason for OnePlus to play marketing tricks here. If the audio recording quality on the OnePlus 3 was good, there wouldn't be any reason to improve it and the company would have focused on improving other aspects of the phone, not the audio. As you can see, or better to say hear from the above video, the sound recording quality is significantly better. The OP3 mic just couldn't record some of the sound range and the result was really disturbing distorted sound. The OP3 just had a very limited dynamic range, which was good for low volume sound I guess. I am not an expect on this, but this is certainly a huge improvement.
I think that many people expect the OnePlus phones to perform well and some where surprised to see such a bad sound recording performance on the OP3. That being said, this belong to the past now. What matter is that OnePlus worked to improve was was seemed to be flawed on the previous model. Maybe some users didn't even notice this in regular video recording, but in some occasions this distorted sound can occur.
It's not that I expect studio recording quality from the OnePlus 5, but I guess that with the new high dynamic range mic, people won't encounter those type of distorted noise issues when recording their videos in situations where this distorted sound can occur.