Raghav,

Raghav Gururajan wrote:
> I have both youtube-viewer and mpv installed. Under preferences 
> of
> youtube-viewer, there is "video_player_selected               =>
> "mpv",". But when I click play in youtube-viewer, the media is 
> not
> opening in mpv. What should I do to make this work?

I can't reproduce this (as it happens, I have both youtube-viewer 
and mpv installed in my regular profile):

============================ Terminal Time 
=============================
~ λ grep video_player_selected 
.config/youtube-viewer/youtube-viewer.conf
  video_player_selected => "mpv",
~ λ youtube-viewer 

=>> Search for YouTube videos (:h for help)
> guix

 1. My crush on GNU Guix (by DebConf Videos) [46:35]
 […snipped…]

=>> Select one or more videos to play (:h for help)
> 1

=>> Description
    […]
===================== MPV opens in a new window 
========================

Are both youtube-viewer and mpv installed to the same profile? 
Which profile is that?  Could you try the command

  guix environment --pure --ad-hoc youtube-viewer mpv -- 
  youtube-viewer

to see if removing any possible sources of confusion helps?  (That 
command also works for me, but I don't *need* it.)

I'm also a bit confused by your phrasing, ‘under preferences…’ and 
‘…click play’.  For me, youtube-viewer is a CLI tool and I've 
always used it as such.  Are you saying there's a gooey?

Kind regards,

T G-R