Actually, players have asked this repeatedly, and Soken has constantly responded saying 'it's not going to happen', due to how music is handled in the game it's not as simple as 'toggling in' 1.0's soundtrack (specifically with zones having various themes playing in certain sections).

Most of 1.0's OST returned anyway, it was mostly the old zone and original battle themes that were scrapped. If you like 1.0's OST that much, buy Before Meteor and listen to it at any time you wish. ARR is a new game, and hence, a new soundtrack accordingly.