What causes the crackling and popping sounds of a wood fire?

Partly settled. Some of it is understood, some is still argued about.

Qualitatively, the crackling and popping of a wood fire is well understood as a combination of several failure processes happening as the wood heats: trapped water and sap boil into steam and burst small pockets in the wood's cellular structure, trapped air pockets expand explosively, and the charring wood itself develops thermal stresses that cause brittle micro-fractures, each releasing a small acoustic pulse. What remains less rigorously modeled is a quantitative, predictive acoustic theory linking a given piece of wood's moisture content, density, and cellular structure to the statistics (loudness, timing, frequency spectrum) of its crackling -- most of the literature on 'crackling noise' as a general statistical phenomenon (a topic studied seriously in fracture and magnetism physics, e.g. Barkhausen-noise-style avalanche statistics) has not been applied comprehensively to firewood combustion specifically, so this remains more folk-empirical than rigorously quantified.

A campfire's crackle is a live audio recording of thousands of microscopic steam explosions and fractures inside the wood.

Explanations that circulated

• The sound is caused only by the flame itself, with no contribution from the solid wood structure
• Crackling intensity depends only on wood species and not on moisture content or seasoning

References

• Avalanches in wood compression, related crackling-noise/acoustic-emission literature in fracturing porous media, arXiv preprint (representative of the crackling-noise research area), 2015