Thanks to an interview from Famitsu with Tetsuya Nomura and Takeharu Ishimoto (composer of Final Fantasy Dissidia NT), we now know more progress is being made on Kingdom Hearts III's soundtrack. Along with Yoko Shimomura, Takeharu Ishimoto and Tsuyoshi Sekito (composer for Final Fantasy VII: Advent Children) are also working on the music for the soundtrack of the game.
An excerpt from the interview about this has been translated by Zephyr from KHInsider:
What do you think about this new information for the soundtrack? Are you a fan of what both Takeharu Ishimoto and Tsuyoshi Sekito have done so far for both Kingdom Hearts and the Final Fantasy series?
Takeharu Ishimoto and Tsuyoshi Sekito Confirmed to be Working on KH III Soundtrack!