I'm just guessing, but it seems you may have a cable problem...perhaps one of the cables concerning the on-board speakers/monitors is defective or is not making proper contact...it is unlikely that the problem would be internal i.e. in the Tyros itself, so I doubt if a system reset will solve the problem.
Have the monitor/on-board speaker cables checked before you do anything else...your speakers are probably okay.
Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.