you pay to talk on xbox live. its one of the features of that service. when you play a game like FF11, you dont play on xbox live, you play on the playonline servers. its as simple as that. you could choose to pay nothing and have the silver package from MS and play only MMO's, or you could pay for the gold service and get that voice chat you value so much but not for MMO's (atleast not FF11). i do see a point however, played fantasy star online at a buddy's house and with all the people talking in the lobby and games, you couldnt make sense of anything. i say with all the MMO's (huxley being the exception) keep it with text only.
only MMO's that were made for the 360 from the ground up would be sutable for voice chat. take True phantasy online for instance. a game like that could incorperate some kind of distance sensitive voice chat (similar to halo2).
basically if a game wasnt built with voice chat in mind (most MMO's) the dont expect it to be included. on the same note, dont expect xbox live features on games that dont run on xbox live but instead the companies independent servers.