i assume you're not trying to do something fancy like use the microkorg for midi, in which case there's lots of reasons why some keys wouldnt make sounds.
in normal situations where you've just turned on the keyboard, you're plugged into an amp or (good) headphones, all the music keys should always work fine, might be worth doing a factory reset. its also worth checking a few things - plugged in properly, power chord in the correct way, amp turned up loud enough, not using busted stupid little earphones. always try to turn it off completely for the count of 30 then back on... to see if it fixes it.
A factory reset should set the software just how it was when oyu started.
It's not likely that this is the problem, but it's good to eliminate possibilities:
first turn off write protect
shift 8 then turn Knob 1 (cutoff) until it says 'off'
then reset factory settings
shift 7 and turn Knob 1 (cuttoff) to 'Prg' which will load all 128 presets again.
and reset global settings
shift 7 and turn Knob 1 (cuttoff) to 'Glb' which is global volume and stuff
if none of the above works then you should really call customer support because i can't think why a healthy synth would behave that way.