While it hasn't been long since Square Enix released its audio drama preview, the Gamescom trailer for Final Fantasy X/X-2 HD Remaster does both audio and visual comparison.  

Beyond upscaling visuals, character models have received upgraded textures and the audio has been remastered.  

You can check out a string of Overdrives, including Yuna summoning the Aeon Valefor to overkill some chump enemies. If you want to jump straight to the new stuff, hit the 1:46 mark of the video.

Are you planning on picking up the HD Remaster?