soundkonverter uses lame for MP3 encoding and twolame for MP2 encoding when present. lame is in Fedora nowadays and twolame is about to be added. Once in, dependencies on those two packages should be added.
twolame is now in rawhide and F27, and a buildroot override is currently in place for the latter.
This bug appears to have been reported against 'rawhide' during the Fedora 28 development cycle. Changing version to '28'.
lame and twolame have been added as weak dependencies. Fixed in soundkonverter-3.0.1-1.fc28.