To reproduce:
1. Have two microphones connected. In Windows, open Sound, Recording, change the default recording device to Microphone1, copy the name of Microphone2
2. In Unity, open user's project, scene TestScene
3. Open script "ED_MicrophoneTest", line 58 (Microphone.Start function)
4. Change Microphone.Start() parameters to Microphone.Start("Your Microphone2 name" + " ", true, 1, maxFreq);
5. Enter play mode, try speaking into Microphone2

Expected: Unity's AudioSource records Microphone2 sounds
Actual: Unity's AudioSource records Microphone1 sounds

Reproduced in: 2017.2.0b11, 2017.2.0p1, 2017.3.0b7, 2018.1.0a2
Not Reproduced in: 5.6.5f1, 2017.1.3f1, 2017.2.0b10

Note: for an easier understanding, changing microphones' names in Windows could help
Note: check image for guides on microphone setup