So, you may need to tell me if I am even trying to do the correct thing. The tracks on this Walkman did not come from my library.
Am I supposed to send/copy/sync all the content of the Walkman to my Library first. Then do any edit, curating, etc. on these tracks in my computer. Then send/sync the files within playlists back to Walkman?

Yes, i cleared the Playing window. But that does not help.

I have discovered two ways to access content on the WALKMAN. One is Walkman/Music/ , the other is WALKMAN/Folders/MUSIC/. The latter is organized in /Artist/Album/Tracks. I can play those tracks although there is no way to display all the tracks at once as I can in the first option. I do not know if I can do other operations on the tracks such as change title, adjust tags, etc.

Have you attempted clearing the Playing and then reloading files from the device?

I am totally mystified. I acquired a new(to me) Sony Walkman. I plugged it in to my Windows USB. It recognized the device, I brought up MM5. The many tracks were displayed. I played several. Thought this is wonderful!

Now while the unit is plugged in and tracks are displayed In MM5, they will not play. I can select a track, but double-click the track or selecting play button does nothing. The playing window displays 947 tracks. I have tried several things: Safely remove device and re-connect. Disable device and enable in device manager, Uninstall in DevMgr (and then reconnect). All with the same results.

I can play from windows file manager. Selecting a track causes the file to be downloaded from the player to an appdata folder and it is played in Media Player Classic (MPC)

Any ideas? Thanks