Sorry, I don’t see any relation between this and my issue.
H) I have some MKV files that the WDTV doesn’t play. Either there’s video but no audio, or vice versa, or doesn’t do anything!
The Matroska specification allows for a form of compression called “Header Removal,” that before recently was rarely encountered. The WDTV Live players do not yet support this form of compression, and may not give an error if it’s encountered.
Recent versions (Version 4.1.0, July 1, 2010 and later) of the popular MKVMerge tool (which is used to create many MKV files) turn this feature ON by default on audio and certain video tracks.
If you suspect that this is your issue (MEDIAINFO will report the Writing Library as MKVMERGE Version 4.1 or later) you can correct it by loading the file into MKVMERGE, then selecting the track(s). Click the Extra Settings Tab and set Compression to NONE. Re-mux the file. This turns OFF the Header Removal Compression and restores the file.
The MKVInfo tool will also explicitly state the form of compression used for each track.