Conversation so far:
Who has he collaborated with?
label-mates The Grace.
Has he had any other collaborations?
Yoochun.
Did he work a lot with Yoochun?
Kim and bandmate Yoochun released a self-composed single "Colors (Melody and Harmony)/Shelter.

Question: What soundtracks did he work on?
Self-contained rewrite: What soundtracks did Kim Jae-joong work on?