As per http://wiki.musicbrainz.org/Live_Sound_Engineer_Relationship_Type the relationship has been deprecated. A script should be run to convert all cases of this into just "Sound Engineer", and the relationship should be removed once it is empty.
Agreed to fix within 12 months by http://scheduling.ocharles.org.uk and http://chatlogs.musicbrainz.org/musicbrainz-devel/2012/2012-01/2012-01-05.html#T19-18-46-411701
In review at http://codereview.musicbrainz.org/r/1718/diff/#index_header
This will be ran when the update is deployed.