Hi
According to the Wiki, CS8406 can be used to add digital audio for any of the mods is listed there.
http://www.gamesx.com/wiki/doku.php?id=matrix (http://www.gamesx.com/wiki/doku.php?id=matrix)
The PSX mod though is not listed on the Wiki, and according to Hojo_Norem in his post on the topic, TC9231N or DIT4096 is needed unless using CS8406 in software mode.
http://nfggames.com/forum2/index.php?topic=3858.0 (http://nfggames.com/forum2/index.php?topic=3858.0)
Are there some other cases of console modding where a TC9231N or DIT4096 are needed rather than a CS8406 ?
Cheers