Does anyone know if the Gold Dragon Feat that changes spell damage die to d8,d10 works with the thundercallers songs? Essentially is the sonic damage and thunder damage songs counted as spell damage?

Thanks!

Thundercaller songs are supernatural abilities and not spells.

The Thunder call ability is worded like this:

Thunder Call (Su): At 3rd level, the thundercaller can use her performance to unleash a deafening peal of thunder. This allows the thundercaller to spend a round of performance to create an effect similar to the spell sound burst (having the same range and area and allowing the same saving throw).

It reads 'an effect similar'. Going by the text description it would mean that Perfect Soul (the Gold Dragon Mythic ability) does not increase the damage of Thunder Call (or other abilities of the Thundercaller's Bardic Performance ability).