During the H scenes of this VN one can hear continuos background voices (usually moans or sucking sounds in the form of looped track). It does not have anything to do with voiced lines of heroines (even if they contains similar moans or sucking sounds).
Do not mix it with Sounds of Copulation - since that tag is about mechanical sounds of friction whereas this tag is about extra voices (also it often has special on/off switch in game's options).