http://tickets.musicbrainz.org/browse/STYLE-193
The "Soundtrack" Work type wasn't created with the idea of having 1:1 mappings with Recordings. This work type is frequently misused in the release relationship editor's "Batch-create new works" feature, but it's not possible to batch-fix the wrong work types.

It would be extremely rare for a new MB Release to actually need to batch-create Soundtrack works. By definition, that would mean that the tracks on the Release are not from the same "film, TV program, videogame, or even book". However, even if the tracks are from different soundtracks, then it is likely that the Release in question is a compilation of previously released music, in which case the MB Works likely already exist.

IMO, there has likely never been a "correct" usage of the "Soundtrack" Work type within the release relationship editor's "Batch-create new works" feature.