Believe it or not, official DVDs of "I, Robot" released in South India sometimes included a Tamil audio track. Check second-hand markets or local sellers. Better yet, support re-releases by studios like Excel Entertainment or Apollo International.
Some fan groups legally produce with permission from copyright holders (rare) or create subtitle files (.srt) that sync with legal copies. Look for non-infringing subtitle repositories like OpenSubtitles.org, which hosts community-translated Tamil .srt files for many films. I Robot Isaidub