According to various sources (like this), FFX features multi-channel audio: Dolby Pro Logic (gameplay) and Dolby Digital 5.1 (movies).
I have my PS2 connected to my surround sound receiver via Toslink, which reports that it's receiving digital 2.0-channel (a.k.a., stereo) in the form of 48k PCM, both in the game and during cutscenes. The receiver is a recent one, which claims to support "Dolby Digital Plus, ProLogic IIx" (among many other formats).
I'm not up on what all of Dolby's brand names mean -- shouldn't my receiver be capable of decoding the PS2/FFX's Dolby bitstream? Do I need to enable them on the PS2 or in FFX somehow? I don't see any configuration options in the game to do so, only mono/stereo.
I know this really won't make any real difference, but I'm curious to see it work!
EDIT: I remembered that there are many different versions of PS2. Does it depend on a particular version of the console itself? I saw Dolby support announced from pretty early sources, and FFX is a fairly early game in the PS2's lifetime, so I don't think it should matter, but FWIW I have an SCPH-55000.