Gordon Wenham has a helpful short discussion on what it means to meditate on the word. This comes from his new book Psalms as Torah.
“It should be noted that neither of the Hebrew words translated as ‘meditate’ or ‘meditation’ refers to silent activities. The verb in Psalm 1:2, hāgâ, also refers to lions roaring (Isa. 31:4) and pigeons cooing (Isa. 59:11), to nations plotting (Ps. 2:1), and to the use of the tongue, mouth, and throat (Pss. 35:28; 27:30; 115:7). The other verb, śîaḥ, is defined as ‘loud, enthusiastic emotionally laden speech’ (e.g., praise in Judg. 5:10; Ps. 145:5; lamentation in Pss. 55:17; taunting in Ps. 69:12) or as ‘meditating with thanks and praise’ (Pss. 77:12; 119:15, 23, 27, 48, 78, 148). When the psalmist speaks of ‘meditating’ on the law, therefore, he is not just thinking about it in his head; rather, he is speaking out loud. He is not reading the text to himself, for several times he mentions doing it at night (Pss. 1:2; 63:6: 77:6), which would have been difficult before the invention of electric light. . . we should probably imagine him singing or reciting the psalm from memory.” (81-82)