I've known about this bug for a while but haven't got around to fixing yet. Is most likely only in V10.011.011 though I didn't test older versions. This topic is firstly to hopefully remind me next time I'm working on coding, and secondly so that people know it's a known issue (and how to work around it).
When attempting to delete a music file from a pack, it will give a "list index out of bounds" error.
Workaround: Save the pack and close Flexi Toolkit. Go into the project folder and delete the music files manually. Now you can open the pack and the musics will be removed.
To investiage: Does this also happen with "Replace"? I know it doesn't happen with "Play".
Flexi Toolkit V10.011.011-B hotfix fixes this bug.