I'm really curious.... if anyone knows... how can flac compress dts? You can't recompress mp3 so why do I get a pretty constant 11% with level 5 compression on dts files. Not looking for a technical answer just a simple one if possible.

Edit: Misunderstood the question.

there's probably some redundancy left because the bitrate of the DTS codec was chosen lower than the data rate.

DTS bitrate on the fake Audio CDs and SPDIF is really 1234 kBit/s (reported by Foobar). This extracted stream cannot be compressed more than around 96%. These 4% come from silence and occasional runs of zero bytes.

The DTS data ends up being white noise because it's essentially random samples. However, if they used all 16 bits then it would be very loud white noise and could cause trouble when it is accidentally played as regular PCM. I guess that for that reason they decided to use only 14 bits, so that it ends up white noise at -12 dB (this is done by setting the 2 MSBs equal to the 3rd MSB.)

Given that data source, FLAC (or any lossless compressor) will save (at best) 1/8 of the data, which is just about what you are seeing. I would guess that a general purpose compressor (like bzip2) might even do better than a dedicated audio compressor, and that the relative performance of different audio compressors might be different than they are with real music.