Problem resolved: apparently the issue was gst-plugins-bad in my system configuration causing problems (which I had hanging around from resolving a website-specific bug in GNOME Web... which I no longer daily-drive anyway).
Thanks for sparking me to actually look into this. I don't know how to close bugs on debbugs or if this one can be considered resolved, so I'll leave that up to the discretion of more knowledgeable Guix hackers. -Juli
