Hello! Thank you for bringing this to my attention. I've had a brief play-about with the Nightly version to see just what's going on. For now from what I can gather it looks like the Nightly Version isn't playing nice with modpak files, presumably Chucklefish are playing around with the way these are handled. For the time being though, here's a link to an unpacked version of the Music Pack; https://www.dropbox.com/s/ps4j174yc9cpax6/Undertale Music Pack.rar?dl=0 File structure there is the same, with the only difference being it's not packed, and this seems to work fine. Enjoy!