there does not seem to be a link to get the firmware.It's posted HERE by Viofo a few replies above this message (#208 in this thread).
I've had voice working properly in this and the previous FW so I'm not sure which one you're on. Seems nothing is different with this FW except it solved the corrupt files when using "auto-detect" parking mode. Low-bitrate still functions well if you prefer using that. Hope the panel-bashers do a good job for you
What was the firmware version ?