This was something I've been seeking to do for some time , in order to
1) tag .dts and .ac3 files used for digital output to an external decoder, and
2) be able to mix 'surround' audio and 'stereo' audio files in the same Foober2000 playlist, for digital output to an AV, wihtout compromising F2K function
Using this thread to report success, finally.
The trick is to process the .dts or .ac3 files with spdifer.exe, which is a component of ac3filter_tools (http://ac3filter.net/projects/tools), a windows command-line tool.
The command line would be
spdifer [input file] [output file.wav] -wav
then compress to .flac (I use -3 because there really isn't much compression going on), tag,and play
This will work, for f2k 0.9x, if using the foo_out_wasapi and foo_spdif plugins. Do not use the foo_dts plugin, that's for DECODING .dts files.