Paul speaks of Christ's word as if it were a person capable of living inside other people. "Word of Christ" here is a metonym for the teachings of Christ. Alternate translation: "Be obedient to the instructions of Christ" or "Always trust Christ's promises" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-metonymy]])
Here "hearts" is a metonym for people's minds or inner beings. Alternate translation: "Sing with thankfulness in your minds" or "Sing and be thankful" (See: [[rc://en/ta/man/translate/figs-metonymy]])