Check the file's tags. They may have gotten erased/not transferred when the files were transferred to the Archos. This program
should be able to allow you to fix them, if they are missing. It's (more likely) also possible the ARCLib got erased, so before checking tags, try this:
- Place your Archos 5 into PC Hard Drive mode (from Settings -> Storage)
- Tap on the Update ARCLibrary button
- Put it back to whatever it was when it finishes
This may take a few minutes depending on the size of your library, but it may fix your problem. If it doesn't, try the tagging software.