I can't remember exactly how it works but; the switches in the front (on the head cover) control the play back heads. In one position you are actually listening to the record heads so that you can sync new parts while recording. Then you switch back to listen to the playback heads (Better quality). If you can't hear all the parts, try the switches in a different position. Make sure that you have all 4 track outputs going to a mixer or monitor system!
Are you getting a signal on the VU meters?
Try one switch one way and the others reversed.... you'll figure it out sooner or later. Anyone out there have a Manual?