I made the new AudioCodex field with an expression so I don't have to manually tag with "Dolby" or "DTS".
If(IsEqual([Compression],truehd,8),Dolby,If(IsEqual([Compression],AC3,8),AC3,If(IsEqual([Compression],dts,8),DTS,)))
Then the DTS trailers I tag the pre-show field with "DTS".
The Dolby trailers I tag with both "Dolby" and "AC3".