Does the additional +1D6 sonic damage from Discordant Voice apply to the slam attacks granted from the Lesser Spirit Totem?
Discordant Voice only adds damage to an attack with a weapon. The damage from Lesser Spirit Totem is not weapon damage, so it does not gain the benefit of Discordant Voice.
There is an editor’s note in the spirit totem page you linked to that specifically states that the spirits are not affected by effects that aid their attack or damage and lists bardic performances as an example. That was from A FAQ that is linked on the page.