When I boot up Duke and move the window to a linked monitor (which is larger than my laptop monitor), the sound continues but the screen freezes. Is there a fix to this? Am i doing something wrong? It works fine in the laptop screen, but not the separate monitor.

I don't think you can just move a game window to a different screen if the program (DOSBox) isn't written to handle it... Try different settings of the Graphics mode setup program and see if any of the settings work. Ultimately try setting up the external monitor as number 1 (with the laptop's monitor as number 2), so that you don't have to move the game window between monitors.