Sansa Clip - computer doesn't recognize music file

Try changing the player’s USB mode to MSC by going to Settings>System Settings>USB. If you still can’t see the files, try MTP mode instead. Files transferred in one mode are invisible to your PC when connected in the other mode (files transferred in either mode are always visible to the player itself, which is why they play even if the PC can’t find them). The third mode, “Auto Detect”, should be avoided as it can change between MTP and MSC between sessions without warning, depending on the whims of Windows.