'К стону... толпы примешивались теперь.. выкрики глашатаев...'

'К стону начинавшей утихать толпы примешивались теперь и были различимы пронзительные выкрики глашатаев, повторявших одни на арамейском, другие на греческом языках все то, что прокричал с помоста прокуратор.'

'Towards/by moaning, beginning-to-fall-silent crowd mingled now-and-was-distinguishable the piercing shouts of the messengers, repeating one in Aramaic, the other in Greek all that had been said from the platform by the procurator.'

Please excuse the roughness of the translation; I'm trying to keep it close to the Russian word order, so that I can get used to how this works.

I can't see why the preposition that starts this sentence is 'К'; obviously the mistake is not the author's, but mine. I don't know of a meaning for this word that translates well into English in this sentence. Would not 'с' or 'в' be more normal here? I would expect my translation should begin 'Within the...' rather than 'Towards/by the...'?

The core of the sentence is выкрики глашатаев примешивались к стону толпы и [выкрики] были различимы.
The grammatical subject is выкрики, and it has 2 predicates: примешивались and были различимы.
"К стону" is a prepositional object to the verb примешиваться, that is normally requires к, not в or с, as well as many other при- verbs (прислонять/прислоняться к, приклеивать/приклеиваться к, прикручивать/прикручиваться к...).

That's an excellent explanation, thank you. I didnt know about 'при-' verbs taking the preposition 'к.'

Shouts are being added to moaning - hence "к"
After shouts have been added - they are within - в стоне, indeed

Yes, that makes sense, if I think of it as 'To the moaning (of the?) crowd, was added...' that makes sense.
i was thinking of 'примешиваться' as meaning 'mixing, alloying', rather than 'adding to'. Slight difference in meaning noted. Thanks