YamahaSynth.com/Soundmondo community:

YAMAHA would like to offer our apologies for disruption to Soundmondo. In an effort to bring you a really cool service for sharing sounds and expand that service to even more instruments, we left ourselves without a rapid response path to resolve issues. As a result, some aspects of Soundmondo aren’t working now but we are aware of them and trying to correct.

Rest assured, we haven’t abandoned Soundmondo. In fact, Soundmondo is a big part of the future for Yamaha Synthesizers.

That said, we can offer some advice that might help some of you. It seems that for Windows users, Chrome version 58 and higher may have issue with sending and receiving MIDI.

If you’re using Windows 10, there’s a workaround that will ease the problem at the expense of Chrome becoming a bit unstable. The fix is:
Flip an experimental flag at chrome://flags/#use-winrt-midi-api to Enabled.
We’re researching fixes for other platforms and will keep you posted as we find them.
Again, our apologies.