Wow, amazing finds once again!
Huh. Not only is this amusingly jank, I'm also surprised at how it means some sounds that were intended to play just never do - particularly those slipup and groan sounds (which relate to small spiders): what was supposed to be a choice of two sounds becomes only a choice of one. The scared scream is a choice of three not four. Rock monster steps, electric fence zaps, raider hup's & haa's, and some other sounds are also affected by this, but they're much less noticeable.
I kinda want to see if I can restore them in Improvements Pack...
This would make a lot of sense - using those various command line parameters you can stop those animations from playing (I particularly remember toying around with the rockfall due to it behaving weirdly on one of my PCs), and it makes sense that the sounds wouldn't play as well.
On boot makes a lot of sense with some of the modding crashes I've encountered. Whilst I've never poked sound modding, I noticed that some invalid model etc files were causing crashes on model load, as opposed to eg any time you actually teleport the model in. Using Community Edition you can enable the -logging parameter which tells you what the game is loading when. I can see that LRR is loading a whole bunch of sound files, like the Catamaran's splash (Rapid Rider), but not the mission briefings - which would make sense with what you've specified and how the Streamed section works.
Fascinating discoveries!