i noticed this in numerous speedruns i was watching & said hey why is northern caves theme being replaced with another soundtrack. is there something i missed

IIRC, doing the midgar skip straight into Hojo glitches the game's music later down the line for the final dungeon.

That's right, the only way to get the music back is by resetting the console after doing the skip, which is slow. The other soundtrack you hear is probably the streamer playing videos in the background.