Hi, I doubt it is a serious matter.
Anyway as per the user manual, it is worth to check the following. With the array of buttons simple things can mute volume
1. The master volume may be set too low. Set it to an appropriate level with the [MASTER
VOLUME] control.
2. All keyboard parts are set to off. Use the PART ON/OFF [RIGHT 1]/[RIGHT 2]/[RIGHT
3]/
button to turn it on (page 38).
3. The volume of individual parts may be set too low. Raise the volume in the BALANCE
display (page 61).
4. Make sure the desired channel is set to on in the CHANNEL display (pages 60, 75).
5. Headphones are connected, disabling the speaker output. Unplug the headphones.
6. If volume control is assigned to the [ASSIGN] slider or foot controller, set it to an appropriate
level.
7. The [FADE IN/OUT] button is on, muting the sound. Press the [FADE IN/OUT] button to
turn the function off.
8. Connect the footswitch to the appropriate jack.
9.Check whether the external speaker is connected and the volume control is adjusted
properly (pages 98, 106).