Bubble is another imitative word, and it’s used to describe the water in a barrel-floating scene and a mysterious sound when Bilbo is approaching Smaug.  Now the barrel scenes are funny and Smaug is pretty high and dangerous but in this particular paragraph, Smaug’s rumblings are likened to the purring of a giant tomcat.  Definitely part of a funny, tension-breaking image, and we’ll tag this word as “low”.

• 10.001 and bubbling.
• 12.011 a sort of bubbling

“bubble, v.” OED Online. Oxford University Press, March 2015. Web. 22 May 2015.

This tinkle comes when Bilbo stumbles over a golden thing, therefore I give it a “low” tag, but the tinkle of broken glass upon the entry of a foe could have a very different register.

• 13.018 and a tinkle

“Thump” is another word whose register depends on what is causing the thumping.  Dwarves threaten to thump plates in Chapter 1 and trolls thump about in a slapstick fight scene in Chapter 2.  While the mood is eerie in Chapter 8, the thumping of feet or of dwarves to the ground are clumsy and low in comparison to the menacing, listening place.  In Chapter 9, Bilbo’s heart thumps during the daring rescue – but their danger is  not particularly great.

• 01.067 Pound them up with a thumping pole;
• 02.070 and thumping,
• 08.001 and the quiet was so deep that their feet seemed to thump along
• 08.105 thump to the ground
• 09.029 Bilbo’s heart thumped every time

“Thud” seems to be an onomatopoeic word the register of which depends on exactly what is doing the thudding.  The feet of oliphaunts on their way to war?  Dangerous and high.  Barrels full of dwarves?  Low.

• 09.050 thudding on top of ones below,

Bilbo’s stammer in Chapter 18 points out the distance between his station and Thranduil’s.

• 18.046 said Bilbo stammering

A squeal is another sound made by prey animals and vulnerable persona when in danger.  Definitely “low”.

• 05.056 Give me time!’ But all that came out with a sudden squeal was: