While you can't officially download just the audio track for free, you can stream or purchase the movie with the Hindi audio included:
The most reliable way to experience the film in Hindi with high-fidelity sound is through official platforms:
This is the current king of piracy. You message a bot, and it streams or sends you the file directly. While convenient, Telegram is riddled with malware disguised as "Infinity War Hindi audio codecs."
Unfortunately, I couldn't find any reliable sources that provide a free Hindi audio track for Avengers: Infinity War. However, I found a few options: