Spirit-Speaker was a title only Solas held among the Evanuris. He carried it with more pride than Fen’Harel and held it in higher regard than even his title of Second to Mythal. It's why it appears first in any of his correspondence and was how he preferred to be addressed, if titles were necessary at all.
I headcanon that this name was given to him by the spirits themselves, as it reflects his deep ties to the Fade and to spirits and his desire to stay connected to his spiritual side. It speaks to what he was known and respected for and why others addressed him as Spirit-Speaker before any of his formal titles as well.
In ancient Arlathan, if a spirit or elf had issues with each other, it was Solas they would turn to.
