I have reported an issue here
But since nobody has a solution I guess this is not my stupidity or a misshaped config file rather than a bug.
Basically the usb audio output randomly disappears. Sometimes it is OK, sometimes it is not. An arbitrary number of restarts solves the problem (sometimes 1st restart, sometimes 4-5 is needed even). But I find rebooting really, really clumsy and slow. So either I would need a configuration that just works, or a handful of commands I can issue to avoid restart but have the system play the sound through the USB yet again.
One more thing: a fresh out-of-the-box flash always works well for a couple of days and the issue develops only after a few days. Then after a week or so I get fed up and reinstall the whole thing, then it works again for a few days until gets bugged. And so on. Therefore I firmly believe this is not a hardvare issue.
Any ideas? I would gladly help to resolve the issue but I’m at lost as I am unfamiliar to the details of how volumio differs from a “typical” raspberry setup.