I have had a look at the book online. I believe you are definitely on the right track. The book was published in 1896, is set in France, and at its start it says "A good many years ago, a young man, student in Paris.....". This makes me believe that it is probably referring to revolutionary pamphlets circulating around the time of the 1848 French Revolution.
Political activities would have been severely restricted at the time, and so the only relatively safe time for pamphlet producers and sellers to circulate their "vehemently factious sheets" would be at night.
Travail means unpleasant and very hard work, and combined with "hum" imply the sound of dissatisfied people, probably in the build up to the revolution.
So, because the man, we are told, doesn't read and is very self-involved, he is unaware of the dissatisfaction and problems of hard-working people which are developing in the country, until he buys the first pamphlet.
That's my understanding of it anyway.
Interested in Language