Performances are being held each Saturday evening beginning this month in the Honmaru Goten on the grounds of Kumamoto Castle. Saturday (the 12th) will be a shamisen performance and next week will feature koto and shakuhachi. All performances begin at 6 PM and last about an hour and a half. There is no additional charge for the performances, but I think you probably need to pay to enter the castle grounds. Performances are scheduled until the end of October.
For information about the Honmaru Goten go to http://www.japanvisitor.com/jā¦/kumamoto-castle-honmaru-goten
Information about the performances: http://www.manyou-kumamoto.jp/contents.cfm?id=711
– Kirk