A video player is a computer program that plays back video files. A 5.1 audio system is a surround sound system that uses five full-range speakers and one subwoofer. The best video player for 5.1 audio is a player that can decode and play back 5.1 audio tracks, and that can output the audio to a 5.1 speaker system.
There are many different video players available, but not all of them can play back 5.1 audio. Some of the most popular video players that can play back 5.1 audio include VLC Media Player, MPC-HC, and Kodi. These players are all free and open source, and they can be used on Windows, Mac, and Linux.