The UserGuide does state “folder” but I agree that it’s most likely a typo.
I created some test files with a video AVI along with a simple audio MP3 (same file names) but that doesn’t work either. If the audio isn’t within the same file (container), it won’t be available. I suppose external sub files get recognised in this manner, so it’s not a leap to expect audio files to be associated when they possess the same filenames…?
As for this feature being “useful for many”… there are probably only a handful of people organising their media this way. Really don’t see a good reason not to have placed all the files within a single container in the first place - who recommended this to you?
MKVmerge GUI can mux them together in a queue, might save a little time? If you get creative with some batch files and do it via CLI, might be able to get it done even faster / leave it running overnight (don’t have the skills to help with that, sorry). How many films are we talking about?
Regardless, the WD TV Live is not the player for you at this time. There’s a new firmware coming “soon” (based on comments on another forum) but if they’re not aware of this ‘bug’ then it won’t contain a fix, meaning you’ll be waiting at least another month (probably more) before you can play a movie with audio.
Your options seem to be:
Mux the audio into MKV (the way they should be tbh).
Return the WD TV Live and get another player that has confirmed support for your odd needs
Contact tech support / create a ticket and get an official response / make them aware of the issue and wait it out.
This community forum is primarily for users to help each other. It’s extremely rare that a WD staff member posts, so complaints and suggestions here are rarely acted upon.
They do allegedly look at the Ideas Exchange though. Might be worth creating a post there too: