GoPro Audio can suffer greatly when you have the action camera in it's cage. However, it's easy to improve GoPro audio and you don't need to buy an expensive mic dongle and microphone to do it.
Get the Adobe Premiere preset here -
You'll no longer have muffled GoPro audio, instead though, you'll have vastly improved audio quality from the GoPro onboard mic that will now sound crisp, clear and give you a great sounding presence
Improving the GoPro audio in Adobe Premiere is done in just a few clicks. And keep in mind, this audio preset is designed for GoPros that are in a cage.