Understanding Digital Audio - "From Audio Device" in Matlab
1) The microphone converts the audio signal to an electrical signal, and then that electrical signal often goes through an amplifier and then to the ADC.  The ADC usually samples the voltage of the electrical signal, which is directly analogous to the pressure of the audio signal.

If you want to think of it in audio terms, probably the most useful way to think of it is that your microphone/amp/something saturates at a certain dB level.  Let's call that "X" dB.  That is where your signal maxes out.  If you have 16-bit sampling then you can span a decibel range of $20log_{10}(2^{16})$ = 96 dB.  That means that, in an ideal world, you could cover audio signals from "X" dB to "X" - 96 dB.  Since the world is not ideal you would probably not get that entire range, but you would probably get pretty close.

2) "Consecutive samples" just means the number of samples.  They throw "consecutive" in there just to say that they are not skipping samples.  If you set that value to 1 you will only get one sample per channel, which is probably not what you want to do.  The number of channels refers to the number of audio sources.  For mono you would set that to 1.  For stereo you would set it to 2.