And since this just happens to be the place on my DVD that is almost worn through, if that is even possible...
And since I’ve been waiting for something special for my 100th post (thanks, Brethil!) You are so welcome!!!!
Yes, I love the way each of the dwarfs is allowed his moment in this scene. We’ve seen them fooling around, we’ve seen them singing even, but looking at that group chowing down around Bilbo’s table is not inspiring; they look like the most rag tag bunch of dwarfs.
They seem to know each other, but I wonder, is this the first time they’ve met since they “got the call?” They’re reaction to Thorin’s knock seems to say “yes” to me. And if so, that puts the song into an interesting light.
As Brethil mentions, there is an overtone of politics in that song; akin to singing a national anthem at a political rally. But, I believe it is more than that. If you read the lyrics, it is obviously a story song, but in the context of the dwarf culture, also a history lesson. Dwarf written language is, from what I’ve gathered, more utilitarian rather than artistic. Their art is in what they make with their hands and what they sing in their songs and therefore an a capella version is most fitting. Story songs are sung when no instruments are available - around a fire, to babies as lullabies, to children as bedtimes stories. So although the singing of that song at that particular moment did achieve political ends (rallying the troops so to speak), it also served as a history lesson for those who have never seen Erebor. I love your point about the Dwarven history lesson Ioreth. That's what I think we see with Dori, moving in and looking so intently and purposefully at Ori (who seems curious yet uninvolved, uninvested in the song). As if he is saying Are you getting this? Do you understand what this all means to us, as Dwarves of Erebor? And that rather oddball and separated trio of brothers come together: a microcosm of the cardinal effect of Thorin singing, and choosing this song, as you say next... This rag tag group who may know each other in passing now becomes a united group with a common goal, a goal rooted deep in the history of their race. They are not only on a quest to reclaim Erebor, but they are following the last of Durin’s line on that quest. Other dwarf settlements exist - the people will not die out - but there is something to be said for reinstating Durin’s heirs to their rightful place.

The sparks up the chimney are a beautiful image, but to me also a very poignant one. I saw two images, the vestiges of dragon fire that they will face and then almost as shooting stars, winking on for a few seconds and then gone. The song was a wish on those “stars”, for success in the endeavor. Like incense, the sparks rise with a prayer to their god(s) for a good outcome. Despite the brave words, I don’t think any of the dwarfs in their secret mind was confident of success or confident in their own bravery in the face of that dragon fire.

Fire as friend or foe - all depends on the origin doesn't it? And perhaps another thing they can quietly yet wordlessly share, getting past any bravado: the dread of that Day, and of the Dragon they have to face.
It’s my view that the sleeping author/historian in Bilbo was being nudged by the dwarfs singing this song. His heart was touched by their plight but I think it was his mind that was awakened by their *story*. Lover of tales, he was primed, as Gandalf saw in the younger Bilbo, to be off on his own adventure. Sam and Frodo talk about being “in the story” instead of telling the story. That is what, I think, ultimately puts Bilbo’s foot over the threshold. This time, he will get to *write* the tale! Very interesting idea into Bilbo's character and motivation. Is that why Gandalf references his 'books and maps'? Recognizing the restless author that Bilbo seems to have grown out of (and into a fussy old bachelor?)