The bowed voice in the low register — the instrument whose range sits nearest the human speaking voice. Aethelred's own, and the sound the Sanctuary hears him in.
Italian violoncello, a double diminutive: violone (the great viol, itself violo + the augmentative -one) plus the diminutive -cello. Literally "the little big-viol" — the instrument smaller than the violone but larger than the viola. English clipped it to cello in the 19th century, keeping only the diminutive ending, so the modern name is a fragment meaning roughly "little one".
Aethelred's own instrument, and the voice the Sanctuary hears him in. The cello's range is the closest of the strings to the human speaking voice — which is why it reads as a person talking rather than an instrument playing.