when the sound is not working in game play does it make some sounds when you turn the machine off then on?
do you still get a thud when you power up the machine or some hum from the speakers
remove the board and check the back of cn2 and 3 for dry / cracked solder joints.
also check and make sure the pins on cn2 are not badly pitted and black.
check that the volume control connector (cn3) on the sound board is also ok

Thanks Andrew, yes there's still the thud when I turn the machine on. The pins on cn2 look fine but I haven't had the board out so ill check that tomorrow.