Lightroom Classic cannot play back the audio from some imported MTS video files.
Reencode your video files to use two or fewer channels of audio. Use another application that can read your videos with 5.1 channel audio and export a copy with only dual (stereo) or single (mono) channel.
Adobe Creative Cloud apps now rely on your operating system to decode or encode Dolby Digital and Dolby Digital Plus audio formats. Adobe no longer bundles the native libraries from Dolby with Creative Cloud apps.
For information about how this change affects Dolby audio playback in your product, see Adobe Creative Cloud apps use native OS support for Dolby.
Lightroom Classic includes a license to decode and play back dual-channel (stereo) audio files. However, it doesn't have that capability for 5.1 surround files.