Japanese term (edited): �A�}�C�N

From various hits on 陰マイク, it appears to refer to two things: one is an actual piece of equipment that cannot be seen by the audience but functions as a microphone - the type that clips on to your tie?, and another is voice of a narrator, for instance, where the person speaking it is never in front of the audience. It seems the interpreter's voice in your case is the latter, where he or she is heard, but not seen. Hence my rendition of "voice-only."