Does the headphone jack work? Replacing a whole board would be expensive and the problem could be elsewhere. What was the accident? If large signals were driven back into the audio out lines, probably the output driver chip is fried... They likely use a common 8 pin dual op amp as the driver and the part costs 50 cents. It takes a little care to replace it as it is likely surface mounted. I am looking at the audio/jack board for a G-600 and there are a pair of transistors for some testing they do that could also be fried and might be the whole problem if the G-6 uses same or similar circuitry. The Fantom X6 uses very similar circuitry... Download service manual here:
http://elektrotanya.com/roland_fantom-x6.pdf/download.html
Scroll down to "get manual" and click to download. IGNORE other download links. On page 57 of 66 you will find the audio drivers at the right. They use little dual op amps M5218APF which probably are available someplace, however probably 4568's would work fine and are readily available. If you drove high voltage into the audio outs, I suspect first that the RF bypass caps such as C203 on the above schematic shorted. Simple to replace... pennies apiece... The test path transistors such as Q37 may also be shorted... Next would be IC18... While this circuit may not be the exact one for the G6 it probably is CLOSE as the several Rolands all seem to use similar circuitry. Regarding system SAFETY... Make sure ALL and I do mean ALL equipment interconnected including mixers, powered speakers, keyboards, etc. are powered from the SAME receptacle or power conditioning module to avoid accidents. Also for keyboards sent to mixers, USE DI boxes please! These protect against ground loops and these accidents.
