OnePlus is one of those companies that likes to slowly tease their products before releasing them. When you think about it, it's pretty clever; fans get more information, and OnePlus gets more coverage. The latest tidbit that OnePlus has 'leaked' about its upcoming iPhone 7 Plus clone is regarding its audio recording quality, specifically in noisy environments. However, it doesn't seem as impressive as the company is trying to make it seem.
Carl Pei, a cofounder of OnePlus, made a forum post talking about how OnePlus 3 and 3T users asked for better audio recording capabilities in loud places. Pei goes on to say that they used new hardware and a group of engineers to produce "much clearer sound," and that the OP5's new microphone "performs significantly better in loud environments."
An unlisted video posted to OnePlus's YouTube channel compares the OnePlus 3's mic with the OnePlus 5's at a concert. While the 3 is unable to properly capture all the bass and distorts the sound, the 5 can. However, the 5's sound does have more echo in general. And to my untrained ears (I'm not much of an audiophile), the mids and highs don't sound as nice.
It's important to keep in mind that since this video was posted to YouTube, the compression isn't doing OnePlus any favors. Also worth noting is that it's possible the 3 just had poor audio recording in general. A comparison with something like the LG V20 or a recent HTC smartphone, or even a Galaxy S8 or Pixel, would be better. However, it might paint the 5's recording in a less flattering light, and after all, this is marketing.