Peter Leithart notes here that, through the person and work of Christ in the cross and resurrection, there is a fourth dimension added to the biblical universe. Previously there was only sea, earth and heaven but in Christ, "under the earth" has been added to the realms which may praise God (Phil 2:9-11). How is this? Only in Christ's overcoming death is the realm of the dead now a realm that may praise God. Leithart implies the answer to the Psalmist's question, "can the dead praise you?" In Christ, the answer is now a resounding YES.
How do those "under the earth" praise God? How do the dead offer up praise? Answer: All those in Christ who have died are not merely dead (John 11:25-26). For the child of God, to die is to be with Christ (Phil 1:21, 23), even if not yet to have a resurrected body - thus, those "under the earth" who are praising God speaks of those who have died and whose bodies are buried but whose spirits have gone to be with Christ (Rom 8:19-25). In his resurrection, Jesus, the first born of many brothers, has triumphed over death and all who are united to him in faith share in his resurrection life. Will we still die? Yes, physically. Our bodies will go under the earth. They will even become earth. But our physical death will not be final just as Jesus' own death was not final. Paul reminds us that, as we died with him, so we live with him (Rom 6:1-14). And Paul reminds us that, even though our bodies go "under the earth", we will one day be raised physically and given a glorious new body (1 Cor. 15:12-49). But for those in Christ, until we receive those glorious new bodies, we will be under the earth praising God.