This verb imitates the sound of what it means: to fall into water or onto a hard surface.

• 01.037 Bilbo plumped down the beer
• 09.049 Heave ho! Splash plump!

“plump, v.1.” OED Online. Oxford University Press, March 2015. Web. 29 May 2015.

The unmistakable sound of Bombur falling in a cobweb out of a tree.

• 08.108 and fell plop on to the ground,

A sound of onomatopoeic origin, and indicating such tiredness that we’re labeling it “low”.

• 07.024 and down dale they plodded.
• 11.004 died away to a plodding gloom.
• 13.065 and they plodded slowly

“plod, v.1.” OED Online. Oxford University Press, March 2015. Web. 27 May 2015.

In case there were any doubt about how serious the scene is:

• 05.007 and a pitter.

This is not a heart attack, just the pattering heart of a gentlehobbit encountering the unknown.

• 05.007 and his heart all of a patter

What a scene!  Giant spiders, Thorin & Company captive, but Tolkien writes it for physical humour:

[08.089] To the fattest of these bundles the spider went – ‘It is poor old Bombur, I’ll bet,’ thought Bilbo – and nipped hard at the nose that stuck out. There was a muffled yelp inside, and a toe shot up and kicked the spider straight and hard. There was life in Bombur still. There was a noise like the kicking of a flabby football, and the enraged spider fell off the branch, only catching itself with its own thread just in time.

• 08.089 There was a muffled yelp inside,

Well, is it low or high?  “Hush” is calming, but it’s also a word of caution.  I like that it’s missing from Chapter 8 through 18.  I’m going to label it “low”, but that does not satisfy: what I mean is that it’s a peaceful word for use in the Shire and among elves except when warning others not to notice a certain giant bear.

• 01.004 and the family hushed it up;
• 01.089 Hush!’ said Gandalf.
• 03.029 “Hush, hush! Good People!
• 03.029 “Hush, hush! Good People!
• 07.133 ‘Hush! Take no notice!’
• 19.014 Hush! Hush! Oak, Ash, Thorn!
• 19.014 Hush! Hush! Oak, Ash, Thorn!
• 19.014 Hushed be all water, till dawn is at hand!

Trust Gloin to say exactly what he means.

• 01.095 Gloin speaking: ‘Humph!’ (or some snort more or less like that).

Owls, dwarves, and the goblins of Chapter 5 do it!

• 02.041 hoot twice like a barn-owl
• 02.042 that he could not hoot even once like any kind of owl
• 02.072 or to hoot like an owl,
• 05.149 hooting and hallooing
• 07.096 and the hoot of owls.