Are the files in GTA V native MP3s? If so, how do the loopable ones actually loop? Like "WDY_NINE_BLURT", otherwise known as "GTA V Score", "GTA V Blitz Play", or "GTA V A Haze of Patriotic Fervor"?
Also, how do the parts actually fit together? For example, within the groups of 8 MP3s, some of them are used for multiple tracks with each track using a different section, as mentioned above with "WDY_NINE_BLURT". IN this case, Blitz Play starts with the 8th MP3 (WDY_NINE_BLURT_8.mp3), and adds I think, #6, or #5 at the second part.
in any case, this is rockstar right, meaning RAGE game engine which definately has that capability, i remember old san andreas game and rockstar did same thing with the tracks, they have cuts and loops, but in ogg format if i remember.
edit: as for how EXACTLY that works, no idea, just some messing around with sound engine should do it. I think Wwise sound engine has that capability also.