I downloaded a movie that has normal sound except for the actors. Their voice is very, very quiet.

It all sounds normal in VLC but I prefer mpv. VLC has an Audio menu item with two options in this file specifically: No Audio and Track 1. It seems Track 1 is selected automatically.

The mpv manual says I can cycle through the audio tracks with the # key, but it seems I can ony cycle through two options: no audio and track 1 except that "Track 1" in mpv has the problem I mentioned, the voices are unacceptably quiet.

Does anyone know here how to overcome this?

Sounds like you are set for surround sound in mpv, but don't have a center channel speaker hooked up. If so, you should set it to stereo, so it mixes everything into just right & left channels.

You'll have to describe your speaker/hdmi/etc configuration and codec and number of audio channels of the video you're playing for more detailed advice...

Bumping this because I still have the problem and maybe someone has a solution.

bumping is against the rules. Posting additional details is not.