Using earthbags for interior walls would take up too much space and so they’re generally not recommended for this purpose. Thinner interior walls are more efficient. The easiest solution is to frame interior walls with 2x4s or, for plumbing walls, 2x6s.
But there are more sustainable alternatives. On some of my plans I use adobe, CEBs, recycled brick or stone around wood stoves for thermal mass. (These materials retain the heat and gradually radiant it back into the house.)
You could also do slipform straw clay (also called light straw clay). I like this method because it avoids sheetrock and the associated taping and texturing, and enables the use of earthen plaster throughout the home. The basic process involves building a recycled or sustainably harvested wood-framed wall 24″ on center, drilling holes and adding saplings through the center of studs, screwing 24″ wide strips of plywood to each side, then stuffing with light straw clay (straw covered with a thin coating of clay slip). Do one course at a time and then move forms up the wall after it has set up. Plaster the walls after drying. This method creates soundproofing between rooms and doesn’t offgass toxic fumes like manmade materials.
Tie interior walls into exterior earthbag walls for strength and rigidity. There are numerous attachment methods, including burying wood blocking in earthbag walls, and using sheet metal anchors.