Hmm... well, editing the sound-track of FFX would just destroy the whole atmosphere. Imagine if you play it for the first time, get tired of the battle themes and install that mod. (SPOILER)All goes well, until the very end when Yuna tells Tidus how much she loves him, while, due to some witty developer's sence of humour, the background music changes to "She smurfing hates me! Tla-la-la".

As for modding, some games, once again on the PC, allow you to do that. You get developer kits and so forth. GTA for one, allows you to load your MP3s so you can listen to them while driving a car -in-game-.