I think the “through the mediation of” rather than “from the singing of” ( if that is what it was) is Harker giving priority to comment over information. Mediation specifically carries the caveats incorporated in Harker’s usage. Singing relies on common sense knowledge that it may not have been carried away correctly or taking up words to say that. Since Harker’s focus is misinformation he embeds that in his jargon term then keeps using it. At least he defines it (sort of) - we have to know what his flavour of Marxism embeds in words like proletariat.