Paste a link in chat to play a video in the theater. Use the console to type "say " and then your link. For example:
say https://www.youtube.com/watch?v=FtutLA63Cp8

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Available video commands
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.host = become the host (only you can control the video)
.skip = stop the current video and load the next one
.stop = stop the current video and clear the queue
.replay = restart the current video, or last video if none is playing
.demo = play the demo video (this changes the video mode)
.queue = show the video queue
.mode = change the video mode (default is "5g")
.fps = change the display fps and restart the video (default fps is "30")
.reliable = toggle reliable audio packets (helps with packet loss)
.sync = change the audio sync delay (default is "350")
.dlight = toggle dynamic light effect (applies to everyone)

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Troubleshooting
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Videos have weird lines/artifacts?
Type "gl_texturemode gl_nearest" in console

No audio?
Audio is played through the microphone system. Try these commands:
voice_enable 1
voice_scale 1
stopsound

Audio is stuttering?
Enable reliable audio packets with the .reliable command

Chat messages are delayed and/or you get "reliable channel overflow" errors?
Disable reliable audio packets with .reliable and lower the video FPS with the .fps command.

Video FPS appears lower than than it should be?
Probably a network problem, but make sure cl_updaterate is at least "30".