Emulator

What's the problem? Describe what went wrong.
I know, this is a known issue and stated in the mario party 7 wiki. But the main issue here, is that only one controller (port 0/1) can activate the mic.
It would be way better to set the input device for the mic activation button freely unrelated to controller port settings.
This way you could set it to "DInput/0/Keyboard Mouse" and map ALL controller xbox/ps4 menu buttons to a certain keyboard input, so anybody could activate the mic without passing one controller around.

//Source/Core/Core/HW/GCPad.cpp
bool GetMicButton(const int pad_num)
{
  return static_cast<GCPad*>(s_config.GetController(pad_num))->GetMicButton();
}

//Source/Core/Core/HW/EXI/EXI_DeviceMic.cpp
case cmdGetStatus:
    if (pos == 0)
      status.button = Pad::GetMicButton(slot);

//it's falsely using the GameCube slot index as GCPad controller index

What steps will reproduce the problem?

Config->GameCube-> Set Slot A to Microphone and go to mic settings -> Input Device will always be the one from Controller Port 0
Config->GameCube-> Set Slot B to Microphone and go to mic settings -> Input Device will always be the one from Controller Port 1

Is the issue present in the latest development version? For future reference, please also write down the version number of the latest development version.

5.0-8449

Is the issue present in the latest stable version?

Yes 5.0

Thanks in advance ;)