Opinion here.Mostly these are original soundtrack excerpts.
There are a few rerecordings. Those which are not original soundtracks, come from Varèse's own batch of rerecordings, however.
Examples:
Korngold's Robin Hood for example is not the OST, but it is Varèse's Utah rerecording.
Jones' Last of the Mohicans is McNeely's RSNO recording.
Williams' Imperial March is Gerhardt's National Philharmonic version.