I have a Casio digital piano and one of the keys sounds louder than the others. Any way to fix this?
You first have to figure out whether it's hardware or software.
First thing to do would be find out how to reset the machine to factory defaults.
I don't know Casios specifically, but this type if thing is usually done by holding some specific combination of panel buttons or sometimes certain piano keys as you power it up. Backup any user-specific data first, if your model has that option.
If that changes nothing, then you're down for taking it apart & checking that the contacts are all even.
Velocity is calculated by measuring the time taken for 2 sets of contacts to be closed. If dirt, grit, crumbs, liquid etc changes that timing, you will hear an unevenness in the perceived velocity.
If the contacts are metal, you will probably achieve a better result by purchasing one or more new contacts. If they are a soft rubberised strip with carbon-ringed 'buttons' then contact cleaner spray will normally do the trick.
Beware that contact cleaner will temporarily soften & distort the rubber, so treat it carefully until it dries & returns to normal.
Don't wipe or scrub it, with anything.