Hello, I have the mod dynamic surroundings installed. It overhauls the SoundSystem. I believe it may be affecting the River Guardian sound. I hear the laser sound while deep underground and it is very loud.

If you are on the current version of the mod, the laser sound should only be heard if there is a river guardian or their target is within 15 meters of you (including you being the target). The river guardians are completely muted with the "isSilent() method if they or their target isn't nearby you (or is you).

If there are any pools or flooded caves near you, albino river guardians are probably spawning in them.

However, I'll test it out with dynamic surroundings and see if it does anything. You could start a creative world and intentionally spawn some guardians in a pool and see how far before you stop hearing them. I think if you are moving away from them, it finishes out the current sound before going silent.