That's not really true. The original web presenter was just a device for converting a SDI video feed into a class-compliant USB "webcam" input. While people frequently use this to feed OBS or other encoding/streaming software, there's nothing about it that is fundamentally tied to streaming.
There is a newer (last year or so) model of Web Presenter that DOES have a built-in streaming capability. But it's still very new and obscure; most people who mention "web presenter" are talking about the original generation.
As for the OPs original question, I'm not sure what the problem is. The audio should come in just like any other webcam audio input. It works fine for me.