Let’s assume you own the original English Avatar Blu-ray or a legal digital copy, and you have obtained a legal Tamil audio track (e.g., ripped from your own Disney+ Hotstar recording for personal backup). Here is how to merge them using free software.
One major problem with downloading audio tracks from unofficial forums (like TamilRockers or Isaimini – which we strongly advise against) is audio desync. The audio might be meant for the theatrical cut, but you have the extended collector's edition. Avatar Tamil Audio Track Download