The main theme of TSD is "
This Is What You Are", which represents the course of the Old War; the Lotus is singing to first gather her children, unifying them before they fight the Orokin, their victory, and the aftermath where she's calling to soothe them to sleep.
The theme of the mini quest,
Apostate, is missing the lead female vocals, the children's voices are no longer unified and discordant and there's an underlying deep hum. This is very much in keeping with the quest, with the hum representing Ballas' disruptive presence, the Lotus missing and the now lost Tenno calling out for their mother.
However with Ordis now synthesising the Lotus' voice, there is a legitimate ingame reason for these
Lotus voice packs.