Complete the indicated chapter, then enter the Options menu to turn on commentaries. Replay the levels from that chapter, find the now available crystals and activate them to hear a commentary from the game developers.

Croft Manor commentaries: Complete Croft Manor
Egypt commentaries (Temple of Khamoon, Obelisk of Khamoon, and Sanctuary of the Scion): Complete Egypt
Greece commentaries (St. Francis' Folly, The Coliseum, Midas' Palace, and the Tomb of Tihocan): Complete Greece
Lost City commentaries (Natla's Mines, The Great Pyramid, and the Final Conflict): Complete Lost City
Peru commentaries (Mountain Caves, City of Vilcabamba, The Lost Valley, and the Tomb of Qualopec): Complete Peru