While looking in the Sounds folder of Rebellion, I came upon a few interesting sound files that I believe are not actually used in-game. The first two sound files relate to the Advent SC- both have a unique On Creation sound file, which is unused. Vasari and TEC SC lack these sound files. Both of the files are present in the original Sins game.
The second group of files are extremely interesting. I'm actually not sure if they're used in-game, as I've had very little time this year to actually play Rebellion. Anyways, there are 11 sound files for a Pirate ship- 4 Attack, 3 General Orders, 3 On Selected, and one Phase Drive file. The sounds are of a stereotypical pirate, but with the same "radio quality" that TEC ships have. The actual quotes are hilarious and utterly ridiculous, such as "AHOOY THAR CAPP'N!" and "TREASURE, YEH SAAY!?".
I just these were pretty interesting files. Go check them out in the Rebellion Sound folder! 