Hi. I've just tried this test: I recorded a short speech and then 'exported mixdown' both in realtime and offline, as you say. I then dragged both mixdowns back into the song (muting the originaltrack). I put mixtool on one track and inverted it. Bingo! No sound whatsoever, not a sausage, even with my system volume on full whack! (Be careful with that, though, folks: if you were to mute one of the tracks accidentally then 'boom'! - could be very loud/blow your cones lol).