The Config file is in the game folder and named Tachyon.cfg.
I found this info on an old Novalogic post, great for setting the audio to what you have.


Go to the Tachyon folder on the hard drive, and double click on the Tachyon.cfg file (use the NotePad to read it).
Find the line, A3D_DEVICE= and change it to one of these settings.
=0 fast 2d (software)
=1 intel rxs (software)
=2 dolby (software)
=3 aureal 3d (hardware)
=4 eax (hardware)
=5 direct sound (hardware)
=6 aureal 2.0 (hardware)
=7 eax 2.0 (hardware)
=8 direct sound (software)
Then SAVE the file.