Missing in the many internet discussions on the propers is the chief reason why we should sing them. And no, that reason isn’t because Rome tells us or provides them for us, or that there is a ready-made choral heritage for them. It’s simply because they are psalms, the prayer of Christ while he was on earth:
355. In praying the psalms of the office for the dead, the assembly offers God praise and intercedes for the deceased person and the mourners in the words of prayer that Jesus himself used during his life on earth. Through the psalms the assembly prays in the voice of Christ, who intercedes on its behalf before the Father. In the psalms of petition and lament it expresses its sorrow and its firm hope in the redemption won by Christ. In the psalms of praise the assembly has a foretaste of the destiny of its deceased member and its own destiny, participation in the liturgy of heaven, where every tear will be wiped away and the Lord’s victory over death will be complete.
Important principles are given here that speak to the broader role of liturgy, and not just of funerals. Praise of God and intercession for those in need: this lies at the core of the funeral rites. Note also the very definition of liturgy: participation in the intercessory prayer of Christ to the Father. Attend to the need to balance psalmody when praying the office of the dead: there should be psalms of both lament and praise.