Eerie sounding instrument used toward the end, while Abby is in her lab, is likely a musical saw rather than a theremin.
When Abby calls for a drum roll before revealing a discovery, the soundtrack obliges her. It's a fitting extra-diegetic sound effect after Abby's close association with a dead scientist who talked to thin air.
Ducky references the Nietzsche quotation "If you look long enough into an abyss, the abyss looks into you." This was used as the closing quotation in the series premiere of Criminal Minds. Meredith Monroe had a recurring role on the series for years. Raphael Sbarge (Mr. K) also appeared in an episode.