Yesterday I installed (like I had a choice) Windows 10 feature update 1809 on the system that hosts all my FLAC files for Sonos. Today none of the clients could access the files. I had to remove the link in the Music Library Settings and replace it. I've had to do this before when Windows updates change something. Don't know if this would have other impacts on Sonos but I thought I would pass it along as I know we have a few Sonos users in this community.
The only time I had a problem like that was when I built a new Sonos server and shutoff SMB v1 support for security. I know SMB v1 is finally off by default in 1809 although I didn't think upgrading it was supposed to change it, so it may be unrelated.
I wouldn't think that would stop the application running on the PC from accessing the music files stored on a local (c:) drive.
I didn't specifically mention that detail.