i have the same problem with mkv files i genrated with handbrake from my dvd iso files.
I used h.264 videocodec and ac3 passthrough as audio track.
When i play those mkvs the audio drifts out of sync by time, i can get it back into sync by rewinding for some seconds.
So i thought ok i try directly my iso files.
In the beginning everything worked fine even ff or rew works amazingly well, much better than my (quite old) dvd player does,
but suddenly the audio starts to switch from one track to another and subtitles go on and off.
1 minute english audio german subtitles, than 5 minutes german audio and no subtitles, and than vice versa.
So no real fun either.
I am a little bit shocked, in what kind of a buggy state this device is currently regarding the firmware.
It cant perform those tasks it was designed for initaly. But hey i could watch some youtube clips
I think WD developers need to do a lot of urgent bug fixing before they should be allowed to see any feature wish lists
with netflix, hulu, usb keyboard, webbrowser or plugin api.
The list of bugs seems to be quite long according to posts in this and other forums.
So please BUGs first!
New Features second.
Sorry forgot to mention i use the 1.01.11 firmware and play from directly attached usb hdd