This is an LMMS 1.2.0-rc2 issue mainly. When you open .mmp or .mmpz files from https://lmms.io/lsp/ and they contain AudioFileProcessor files, it mostly does not autodetect any of these files in your LMMS directory. This needs to be fixed as many songs will use <yourfilepath>/data/samples/drumsynth/ files, will nearly all of them will go missing through auto-detect and then viewing the error message with missing files that you have in your directory.