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pacmd can be cumbersome to use, if you intend to control pulse state there are a few more "elegant" CLI mixers like ponymix or pamixer or so.
FWIW the same general process in pacmd terms would be
pacmd set-default-sink alsa_output.usb-SteelSeries_Arctis_Pro_Wireless-00.analog-stereo #Ensure future applications default to the stereo sink
pacmd move-sink-input 25 alsa_output.usb-SteelSeries_Arctis_Pro_Wireless-00.analog-stereo #Move mpv to the stereo sink, that 25 is based on the sink-input index above that will be a different number for each application playing audiobut if you aren't really fluid with what the pulse terms represent pavucontrol provides a nice general visualisation of how this looks like.
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