An experiment was carried out to estimate the effect of sound feedback on the work of a plant operator. Eight students of computer science operated a process simulation program of an assembly line with computer numeric controlled (CNC) robots. Relevant information of disturbances and machine breakdowns was given only in a visual (test condition 1), and in visual and audible form (test condition 2). The results indicate, that the additional sound feedback improves significantly the operator performance and increases positively some mood aspects.