Mirillis forum

why can't i turn on both audio tracks when uploading videos to youtube?

YouTube only supports one active audio track per video. If you upload a video with multiple audio tracks, YouTube will typically use the default track. To include multiple audio options, you can:
Merge Tracks: Combine the audio tracks into one using video editing software.
Upload in Multiple Languages: Use YouTube Studio's "Captions" or "Audio" features to add alternate audio tracks for language purposes (if your account supports it).

YouTube only supports one audio track per video. Combine tracks in editing, upload separate versions, or use subtitles for multiple language options.

Even though you can upload videos with multiple audio tracks, YouTube only allows one audio track to be active during playback. This is by design because YouTube’s player is not meant to mix or blend multiple audio tracks simultaneously.

You cannot play both audio tracks (e.g. background music + voice) when uploading a video to YouTube unless they are pre-mixed into the same audio file in the video. When you upload a video, YouTube only plays one main audio track. It does not have the ability to play two separate audio tracks side by side like editing software does.