Here's a little more detailed breakdown of stereo vs. mono:
Frist, there are channels and there are tracks. A channel is a path of signal flow through a mixer or other audio device. It has an input and an output, or an input with a selection of outputs. A track is a recording path, and though the term dates back to the days of analog when the tape was divided into separate tracks under a record head, it carries on now in digital. Typically, with the use of a mixer (or if your recorder has built-in routing controls), any input channel can be recorded to any track.
On the other end, at playback, any recorded track can play back through any output channel (in both stereo and surround systems) by using pan control.
Two-track audio recorders usually default with the input channels matched to the record tracks. So, channel 1 records to track 1, and channel 2 to track 2. So, is it mono or stereo?
Mono vs. stereo isn't just a matter of one channel or track vs. two. It has to do with how two channels or tracks interact with eachother upon playback.
Mono audio comes from a single input channel. It may be recorded to one or more tracks. In post, only one of the record tracks is typically used, and that track is adjusted using the pan control to play either equally from both left and right speakers in a stereo system, or from the center channel of a 5.1 or 7.1 surround system. Mono means that it comes from one place.
Stereo requires two channels and two tracks, but has to do with how the two paths interact. Two recorded tracks, playing back across two output channels, work together to create a spatial image. Or, many recorded tracks that are panned left and right, may come together into two playback channels to create an image with a left-right spatial spread. Why does this matter? Well, if you have two actors, each wearing a lav with wireless transmitter, recorded to two separate tracks, that is NOT stereo. It is dual mono, as each source will be panned center in post. The two are not intended to create a left-right spatial relationship. 8 actors on 8 wireless mics recorded to 8 separate tracks? That's multi-mono.
This gets confusing because many audio recorders flag 2-track recordings as a stereo pair no matter what, and stereo has become a default term for two linked tracks in many NLE and DAW platforms. Just remember, mono vs. stereo greatly depends on how the audio was recorded and how it is intended to play back. So, if you record one mic to both tracks on your DR-100mkII, you will want to split the stereo pair in your NLE and use only one track, panned to the center.
As for microphones, most mics are mono, though one could argue that ALL mics are mono. There are stereo mics out there, but those may be better referred two as stereo mic arrays because each one actually contains two mic capsules (each capsule is mono) mounted in a single chassis. In other words, a stereo mic is basically two mics housed in one body.
If you buy a shotgun, hypercardioid, or lav, it is going to be mono unless you purchase a model that specifically says "stereo."
Dialog is a mono source 98% of the time. The other 2% applies to special edits where a voice may be heard off-screen and the director wants that off-screen person's location to be obvious to the viewer. Because mono and stereo are about spatial directionality in playback, and dialog is almost always uni-directional (from the center of the mix), it is recorded as such.
Further, recording dialog in stereo can create its own host of challenges. Not all stereo mic arrays are mono-compatible, meaning if both tracks are panned center and played back together there can be artifacts like phase cancellation. The movement of the actors also means that one of the mic capsules in the array will usually be off-axis, and it won't be consistently the same mic capsule, so ditching one in post and using the other will result in inconsistent sound. Also, recording the dialog in stereo and leaving it that way throughout post-production and into screening will create a very disorienting movement of the dialog that can be hard on the viewer.
The final stereo mix of a film will be a mixture of mono and stereo recordings. Things like dialog, Foley, and many FX will be mono sources. Things like music, ambient sound beds (background sounds), and some FX will be stereo sources. But even though they are all mixed together into a stereo mix, all the mono sources are panned to the center.
Likewise in surround mixes, there will be a mixture of mono, stereo, and surround sources. The mono sources will be routed to the center playback channel. The stereo sources will be routed to front left and front right, or may be layered with others to that are sent to the rear fills to create a surround spatial image. And surround sources (usually ambient beds and some FX) will be sent to front L/R and rear L/R.
