All fixed! Had to re-install Realtek audio driver direct from HP (for second time)
Issue came up because of System Restore (BSOD) and having to re-install Security Updates, take it to be one more Windows 10 Pro v.1703-caused conflict .... but (as before) built-in mic & speakers (HP 840 EliteBook G2) AND headset (incl. mic) work with Audacity (easy way to check) and with Skype.
Since MS owns Skype, you'd think they'd have figured this out beforehand
... but "the System" worked (HP's part) and "All's Well That Ends Well" (so far)