If I understand correctly, MTP mode is broken on your PC, and the the device is reverting to MSC mode.
No worries, you can use MSC, but you lose the background functions of MTP. Depending upon your audiobook source,MTP is required for the DRM (license) capability.
I believe Audible can be used in MSC, as their DRM is a little different.
To fix the MTP system (if the device communicates in MSC, you’re almost there, as data is making it to the device), manually select MTP mode to keep the Auto Detect from reverting to the other mode, and plug in.
If you open the Device Manager, find the Clip listed under Portable Device. The Device Manager can be found by starting at the Control Panel. Double click on the Sansa, or right click, and select Properties > Driver > uninstall. I like the shortcut right click directly to uninstall. easy huh? Then, unplug the device, wait a moment (30 seconds should suffice), then plug in again, and it will reinstall automatically).
Remember, the MTP drivers are part of Windows Media Player 10 and later. Windows XP came “out of the box” with WiMP 9, so that can be upgraded by simply installing WiMP10. The Microsoft MTP Porting Kit application is available at the Microsoft website too, quite often installing the Kit repairs a frazzled WiMP 9 and later installation.
Why does MTP disappear? Quite often, the registry entry for the Sansa is the key, as MTP capability is simply “turned off”. The registry is fragile, often modified as you plug in various things on those USB ports, or install different applications. Windows uses the registry as a handy cheat sheer to decide how to handle devices found on the USB bus. If the entry is corrupted, we suddenly lose functions like MTP. Try the reinstall trick, it is the most basic fix, and usually solves the issue.