The way I see it, using whispers is the best way for voice input because:
1. It automatically keeps things private.
2. The computer can differentiate between
instructions to itself and dialog in the
"Computer - record this conversation.
Execute HelpScript1, if EscapePhrase1
detected or if gunshot detected."
*Computer audio in earpiece*
** Now You speak normally **
"There you Marco - how's the underworld
doing today? "
A combination earpiece - a tiny speaker + a
bone conduction microphone - could be used for this.