Maybe not the same problem as your DC2M4, but my speaker volume also wasn't working either on my Mark IV (volume remained low). I ended up replacing the amplifier board in the controller (item WC704701, PANEL CIRCUIT BOARD MK4, Yamaha service (714) 522-9011 ). I replaced the board myself, but the part alone was $600. Not cheap, but 1/2 cost the local dealer was asking for the same repair. It wasn't hard, but I have some experience with building my own PCs.