You might want to try a newer version which should cure the lack of sound on some of your MKV’s. I think around 1.03.43 they added compatibility with compressed headers, which had been causing players to lock up or not play the soundtrack of some files.
http://wdc.custhelp.com/app/answers/detail/a_id/5800
I’ve updated to 1.04.22 - the last release which has given me no problems. You can install any firmware, and even roll back to a previous version. This version also writes resume points to the USB drive, which is pretty nice.
