A lot of KH3 music gets copyright flagged so if they are using twitch or youtube it can get muted. If this is the case then it will happen to mostly cutscenes and is especially prevelant on Arendelle and major story cutscenes like the intro. If not these platforms then it is likely caused by the weird sound channel issues (I don't know enough about the game's design to truly know if they use several different sound channels to layer sounds but considering how many people have "I can here this but not this" issues it sounds like some channels are bugging out at times).

Basically what sound channels/tracks do is instead of playing every sound in one recording you're actually hearing several different recordings playing on top of each other (music is recorded/played this way). It lets them adjust things like volume independently of other sounds.