Need Help: Is Audacity Auto-mixing? And dual mics?

Hi, new to audacity, and have a question, aswell a problem I’ve been experiencing.

I want to attempt a podcast, and I’ve come to terms with having to use my low quality mics, but I have a few problems.

First of all, I am unsure whether I am able to use both my laptop’s in-built microphone AND a line-in mic I have, simultaneously. Both work seperately but I dunno if I can use them both at once.

And secondly, I’ve been having a weird problem. I’ve been sound checking different audio devices that have mics in them. On first use, I can get a good sound out of them (i.e it makes a nice large pattern on the waveform, and I can hear myhself clearly when I play it back), but then, when I attempt to recreate that, I find that when I have the mic the exacts same distance away, and without changing any settings, it’s like the mic can’t hear me. If I tap the mic, or speak loud enough it will eventually register sound, but only as VERY small on the waveform, almost unhearable upon playback. I can hear through my speakers the microphone is picking me up as clearly as before, but Audacity is not. Very annoying.

Hope you can help me guys.

Which version of Audacity? (1.3.14 is recommended)
Which version of Windows (or other operating system)?

Probably not. The sound card drivers usually allow you to chose either the internal microphone OR an external microphone but not both at the same time.
Audacity can only select one “device” (sound card) for recording at a time so you also cannot record from a USB mic at the same time as the computer’s internal sound card.

That sounds like a variation on this issue:

Why do my recordings fade out or sound as if they were made in a tunnel

Also, ensure that you do not have Skype (or other voip program) running in the background. Skype is known to change the sound system settings without any indication that it is doing so other than the recording level suddenly changing without warning.