The Hebrews considered the bowels and internal organs the seat of the emotions, in the same way we speak of our hearts today. Isaiah is therefore expressing his grief intensely. As Isaiah declares only what God commands, we know that God is also moved by Moab’s plight.
However, the reason for this even more intense grief is not the destruction of Moab, but their reaction to it, as seen in the next verse.
11 Therefore my inner parts moan like a lyre for Moab, and my inmost self for Kir-hareseth.