Well to be fair this was 1999, and back then having mouth animations on 3D character models at all was enough to make people go "whoa!" Granted I would say Conker's Bad Fur Day on the N64, a "weaker" console, did have better mouth animations than Sonic Adventure 1 did (though again to be fair, CBFD came out two years after Sonic Adventure 1, so they had time to go "here's what this game did, now lets see if we can do better").

Heck even in the realm of PC gaming having mouth movement animations was something of a rarity back then too. Half Life 1 uses anime-like lip flaps, Thief 1 and 2 had faces that didn't animate at all on the models and the cutscenes rely on a combo of silhouetted live action actors and "motion comic" styled animation, and while true Deus Ex did have characters with lip movements while talking, the lip movements don't always sync up to the words being spoken.