Question How to make a low pass filter to be active when the player enters a triggerbox collider?

I would probably use AudioMixer Snapshots for this.
With the Mixer, you can send all the sounds you want to be influenced by the low-pass on an AffectedByWater MixerGroup (sfx, room tones, weather, ...) and keep ambiances and music on other groups.
Then you make a MixerSnapshot for AboveWater and BelowWater that adjusts a low-pass on the AffectedByWater MixerGroup and toggle between them when hitting your water collider.