Most likely, the video is the Extended Cut (178 min), while the audio is from the Theatrical Cut (162 min). Solution: Find a version that matches, or use editing software to slice/expand the audio.
The most reliable and high-quality way to experience Avatar with Tamil audio is through official streaming services that hold the distribution rights. These platforms provide professional dubbing and multi-channel audio (like 5.1 surround sound) that are difficult to replicate via unofficial downloads. Avatar Tamil Audio Track Download