I haven't found a song just like Raise Your Voice, which sounds very unique with three vocalists and the simple but lovely instrumentals in the background. I don't know of an OST like it. But here are some suggestions that I think are similar to what you are looking for:
It's You by Henry (While You Were Sleeping). It starts pretty slow and seems sad, but gets stronger throughout and overall feels very uplifting. It's not quite as Disney-esque but definitely fits the "ML saying everything will be okay" vibe.
I Hope I'll Reach You by Maktub (School 2017). I think it has a Disney vibe and and like It's You^ it starts soft and sad, but grows stronger and the chorus feels very hopeful.
These songs are not sad but might be too calm:
Maybe by Davichi (Her Private Life)
A Little Bit More by Jinho and Rothy (What’s Wrong with Secretary Kim)
^^This one feels more like a Disney song mostly because of the male and female vocals.
Others that I thought of but might be too upbeat:
I'll Pick You Up by Standing Egg (Weightlifting Fairy)
I Pray 4 You by Apink BnN (School 2017)
I hope you like one of these! Good luck with your search :)
