The 'hum' of conversation
“A rumble of men's voices” might be an appropriate phrase, using rumble's sense “A low, heavy, continuous sound, such as that of thunder or a hungry stomach”.
OP could go with murmur, but I and most other Anglophones tend to use buzz...
On the other hand, since OP wants a word evocative of running water, it's worth noting...
babbling brook 44,500 instances in Google Books
babble of conversation 24,900 instances
Burble
To make the quiet sound of water moving over rocks
also purl
To flow or ripple with a murmuring sound.