One way to confirm whether the files are really just MP3 files is to open them in QuickTime and then open the Inspector (hold ⌘ and tap I or choose from Show Inspector from Window menu). The Inspector will tell you the file type (if it recognises it).
If these are MP3 files it should say something that starts with "MPEG Layer 3" for the format. If it doesn't then either it isn't an MP3 file, or it is but one that is corrupted in some way.
Assuming these are simple MP3 files, having looked at other reports of people getting the same error message, have you tried removing the files from your iPhone and syncing them back over again. It seems people can get this error if the files are corrupted while syncing.