if you really have a version 5, you do NOT have a Yamaha sound chip.
you have a AD1881A made by Analog Devices. it is the black chip (U18) about an inch from the back of the keyboard connection, to the left of where the flash memory card socket(CN10)would go. if you have the Yamaha chip, it would be about a half inch off the pin 26 end of CN10 at U14.
if you are using the drivers for the Yamaha chip but have the Analog Devices chip, that would explain your sound problem.
if you were running windoze, you would need the VIA sound driver, but i do not know what driver to use for your system. the driver Analog Devices has for the AD1881 does not work under windoze (which makes me think the AD chip is not really used, but the sound section of the VIA chip is used instead).
are you the original owner of this i-o, or did you get it used? if you got it from someone beside netpliance, did they try to hack the sound? did anyone ever short out the speakers?
U10, next to the right speaker connection is the audio amplifier chip LM4835M. if that is shot, that could also explain no sound.