Extraneous sounds, picked up by a microphone. Generally used to describe the unwanted sound of one musical instrument as heard by a microphone in front of another instrument.
"Just a short note to thank you for the wonderful job you did transferring my old cassette of Japanese koto music to CD. I haven't be able to find a CD version of this tape in Japan and had stopped playing it to avoid wearing it out. Within days of sending it to you I received back an excellent CD of this beloved tape at a very low cost. Thanks ..."