Psalms 40:6

Animal and meal offerings were not of primary importance to God under the Mosaic Law. More important than sacrifices for either worship or expiation was the believer’s true commitment of himself or herself to the Lord (cf. 1 Samuel 15:22-23).

The phrase, "my ears Thou hast opened (or pierced)," may mean David viewed God as having made him His willing slave by being so gracious to him (cf. Exodus 21:6). However, it seems more probable that David meant God had given him the ability to comprehend and obey His Word (cf. Psalms 40:8).