It could be a tough problem to solve. I don't normally respond to DLNA related questions, but I am interested in zone synchronization as well, so I thought I would chime in for what it's worth.
Since one of your outputs is DLNA and the other is not, the problem may be intractable. You would have a much greater chance of success if you could run direct to those speakers somehow instead of DLNA. The analog output will play exactly what MC says, exactly when it's told to do it. The DLNA device will operate, to an certain extent, on its own schedule. There will always be some latency there that is not controllable.
You give a lot of info about your PC, but the one thing you left out is what the "wifi speakers" actually are. Do you have any other means of getting a signal to them besides DLNA, like either a cable or a proprietary wireless driver (like KEF had)? EDIT: I found your other post, where you say you have the Hitachi W200. You will probably achieve better synchronization with less latency, albeit with worse sound quality, if you send to the W200 via Bluetooth instead of DLNA. (If you're in range that is. Check into it at least.) That speaker also has a 3.5mm aux input, so you could run a cable to it, or you could hook it up to a better low-latency wireless audio transceiver. Such devices exist. All those options would outperform DLNA.
Hopefully someone else might come along with better ideas, but I can only offer a couple of things...
1. Read the recommended steps in this post and apply them:
https://yabb.jriver.com/interact/index.php?topic=115328.0If you have to deviate from any of them, report that so that people know what you're doing.
2. As is mentioned briefly in that article, output at the same sample rate to both devices. Your wifi speakers probably resample everything, so you might need to learn what their internal resampling rate is, output at that, and do the same with your analog output.
You might think unnecessary resampling is not ideal for audio quality, and you'd be right, but in this case synchronization takes precedence and you can get a little bit of drift due to different clock rates. That drift will increase over time, which sounds like what you're describing. So making sure you're running at the same sample rate on both outputs will minimize that to the extent possible.
3. As I said above, try Bluetooth to your speaker instead of DLNA. It will probably be much more responsive that way.
I'll mention that when I did a similar stunt a while back, I ended up running quite a long audio cable through the crawlspace under a house in order to get a direct connection to some speakers, because the complications (like what you are seeing) from trying to do one zone wireless and keep them in sync were just too much. It was a pain running that cable, but I've never regretted it.
Anyway, that's all I have. Good luck, and I'll be interested to hear how well you can do.