Updates in bold
-Verify mic works/configured correctly in Steam, Windows, and every other audio-input utilizing application.
-Once game is launched, open up Steam Overlay—>Friends List Settings—>Voice.
-Notice during a mic test with Push-to-Talk or Push-to-Mute enabled that not input is recognized (if "short sound on mic" is toggled, that sound will not play anymore either). (mitigated by ensuring steam has elevated privilege over insurgency, see replies)
-Open Microphone test DOES work but the game has no open-mic option (also, the Push-to-Mute Toggle for Open Mic doesn't work either //elevate steam privilege); you must use a Push-to-Talk keybind.
-Change Steam Chat Push-to-Talk/Mute keybind to match the same in Sandstorm or differently—neither makes a difference (Sandstorm will always use its keybind regardless, as is verifiable by the Team&Promixty or Proximity popups in the lower left corner in-game).
-Verify in game text-chat that in Sandstorm space, no one can hear you scream. (may or may not be mitigated by adjusting applications' privilege)
-Launch any other audio-input utilizing app (e.g. VoIP) and notice/verify they still receive mic input.
Yes I know there's an abundance of threads on the mics not working only in Sandstorm ever since Steam rolled out its new Steam Chat features (a whole year has gone by w/o a fix), but none mentioned this specific problem/observation.
Yes I've tried an abundance of troubleshooting:
-change Steam Chat Input Device
-tweak Steam Chat's various voice settings (don't get on me about gain—every available location is maxed out)
-tweak Windows mic&speaker settings
-tweak audio card settings (yes the drivers are up to date)(Sound Blaster discreet and Realtek on-board audio cards)
-verify game integrity cache
-used different mics and/or headsets (mic&speaker TRS 3.5mm(separate TRSs and TRRS to Y-TRS))(Logitech & SteelSeries headsets) USB-only or adapter works
-set Insurgency.exe, InsurgencyEAC.exe, steam.exe, and GameOverlayUI.exe to run as admin, in varying combinations; having at least Insurgency.exe and steam.exe run as admin gave my setup the most functionality towards mitigating the issue, barring other apparent input issues of course.
I'm on Win7 and notice its "Set up microphone" for Speech Recognition is visually nor audibly helpful and wonder how that configuration goes on Win10. I also notice that once the game is running and the Steam Push-to-Talk/Mute is disabled that the Overlay will not let me rebind to my secondary (Logitech) mouse keys—it will still take the primary mouse-keys as bindings—even though the game (and everything else) still recognizes all of my mouse's keys. I've also noticed that under some unknown but likely specific circumstances that the game stops recognizing certain keystrokes/bindings while in the Settings menu—namely the Esc key to exit the menu, and the F# keys to Apply and Reset settings; I have yet to isolate that issue whether its related or not.
I'll update the OP with more troubleshooting and reply as I try more things; next on the list is reinstalling the game...