Immediately after a crash and forced poweroff, I ran "journalctl -b -1", which I've pasted below (minus the huge amount of systemd-udevd logs which are likely irrelevant).  The freeze always happens in VLC, and indeed the logs show VLC failures at the end of the logs.

I should also note that I'm using glitchy cheap chinese bluetooth speakers.  No matter what device connects to these speakers (phones and various laptops), the sound often dies and I have to disconnect and reconnect the bluetooth session to recover.  I doubt that's related but mention it for completeness.

When VLC can bring down the whole platform, that seems like a serious problem with the kernel.  Regardless of how badly an app like VLC behaves, it shouldn't be able to freeze the whole system.

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Aug 21 08:53:40 hostname kernel: microcode: microcode updated early to revision 0x368, date = 2019-04-23
Aug 21 08:53:40 hostname kernel: Linux version 5.4.0-42-generic (buildd@lgw01-amd64-038) (gcc version 9.3.0 (Ubuntu 9.3.0-10ubuntu2)) #46-Ubuntu SMP Fri Jul 10 00:24:02 UTC 2020 (Ubuntu 5.4.0-42.46-generic 5.4.44)
Aug 21 08:53:40 hostname kernel: Command line: BOOT_IMAGE=/boot/vmlinuz-5.4.0-42-generic root=UUID=(redacted) ro quiet splash vt.handoff=7
Aug 21 08:53:40 hostname kernel: KERNEL supported cpus:
Aug 21 08:53:40 hostname kernel:   Intel GenuineIntel
Aug 21 08:53:40 hostname kernel:   AMD AuthenticAMD
Aug 21 08:53:40 hostname kernel:   Hygon HygonGenuine
Aug 21 08:53:40 hostname kernel:   Centaur CentaurHauls
Aug 21 08:53:40 hostname kernel:   zhaoxin   Shanghai  
Aug 21 08:53:40 hostname kernel: x86/fpu: x87 FPU will use FXSAVE

(...)  [copious systemd-udevd logs snipped]

Aug 21 09:19:11 hostname systemd-udevd[4206]: value '[dmi/id]sys_vendor' is 'Dell Inc.'
Aug 21 09:19:11 hostname systemd-udevd[4205]: ext4_inode_cache(357:console-setup.service): Device (SEQNUM=7613, ACTION=remove) processed
Aug 21 09:19:11 hostname systemd-udevd[4205]: ext4_inode_cache(357:console-setup.service): sd-device-monitor: Passed 164 byte to netlink monitor
Aug 21 09:19:11 hostname systemd-udevd[4206]: ext4_inode_cache(1161:NetworkManager-dispatcher.service): Device (SEQNUM=7614, ACTION=remove) processed
Aug 21 09:19:11 hostname systemd-udevd[4206]: ext4_inode_cache(1161:NetworkManager-dispatcher.service): sd-device-monitor: Passed 177 byte to netlink monitor
Aug 21 09:19:14 hostname systemd-udevd[325]: Cleanup idle workers
Aug 21 09:19:14 hostname systemd-udevd[4205]: Unload module index
Aug 21 09:19:14 hostname systemd-udevd[4205]: Unloaded link configuration context.
Aug 21 09:19:14 hostname systemd-udevd[4206]: Unload module index
Aug 21 09:19:14 hostname systemd-udevd[4206]: Unloaded link configuration context.
Aug 21 09:19:14 hostname systemd-udevd[325]: Worker [4205] exited
Aug 21 09:19:14 hostname systemd-udevd[325]: Worker [4206] exited
Aug 21 09:19:23 hostname vlc.desktop[2577]: [00005591dba78700] main audio output error: too low audio sample frequency (0)
Aug 21 09:19:23 hostname vlc.desktop[2577]: [00007fb34d0176b0] main decoder error: failed to create audio output
Aug 21 09:19:23 hostname vlc.desktop[2577]: [00007fb34c009260] ts demux error: libdvbpsi error (PSI decoder): TS discontinuity (received 11, expected 10) for PID 0
Aug 21 09:19:23 hostname vlc.desktop[2577]: [00007fb34d01d6b0] main decoder error: buffer deadlock prevented
Aug 21 09:19:23 hostname vlc.desktop[2577]: [00007fb34d0176b0] main decoder error: buffer deadlock prevented
Aug 21 09:19:34 hostname pkexec[4257]: pam_unix(polkit-1:session): session opened for user root by (uid=1000)
Aug 21 09:19:34 hostname pkexec[4257]: blee: Executing command [USER=root] [TTY=unknown] [CWD=/home/blee] [COMMAND=/usr/lib/update-notifier/package-system-locked]
Aug 21 09:19:35 hostname vlc.desktop[2577]: [00005591dba78700] main audio output error: too low audio sample frequency (0)
Aug 21 09:19:35 hostname vlc.desktop[2577]: [00007fb38101ca10] main decoder error: failed to create audio output
Aug 21 09:19:35 hostname vlc.desktop[2577]: [00007fb3800050e0] ts demux error: libdvbpsi error (PSI decoder): TS discontinuity (received 11, expected 10) for PID 0
Aug 21 09:19:35 hostname vlc.desktop[2577]: [00007fb38100b580] main decoder error: buffer deadlock prevented
Aug 21 09:19:35 hostname vlc.desktop[2577]: [00007fb38101ca10] main decoder error: buffer deadlock prevented
Aug 21 09:19:41 hostname vlc.desktop[2577]: [00005591dba78700] main audio output error: too low audio sample frequency (0)
Aug 21 09:19:41 hostname vlc.desktop[2577]: [00007fb35d018870] main decoder error: failed to create audio output
Aug 21 09:19:41 hostname vlc.desktop[2577]: [00007fb35c003480] ts demux error: libdvbpsi error (PSI decoder): TS discontinuity (received 11, expected 10) for PID 0
Aug 21 09:19:42 hostname vlc.desktop[2577]: [00007fb35c006f30] main decoder error: buffer deadlock prevented
Aug 21 09:19:42 hostname vlc.desktop[2577]: [00007fb35d018870] main decoder error: buffer deadlock prevented
Aug 21 09:19:46 hostname vlc.desktop[2577]: [00005591dba78700] main audio output error: too low audio sample frequency (0)
Aug 21 09:19:46 hostname vlc.desktop[2577]: [00007fb3a4094a70] main decoder error: failed to create audio output
Aug 21 09:19:46 hostname vlc.desktop[2577]: [00007fb3a40159e0] ts demux error: libdvbpsi error (PSI decoder): TS discontinuity (received 3, expected 0) for PID 0
Aug 21 09:19:46 hostname vlc.desktop[2577]: [00007fb3a40159e0] ts demux error: libdvbpsi error (PSI decoder): TS discontinuity (received 14, expected 11) for PID 80
Aug 21 09:19:46 hostname vlc.desktop[2577]: [00005591dba78700] vlcpulse audio output error: digital pass-through stream connection failure: Not supported
Aug 21 09:19:46 hostname vlc.desktop[2577]: [00005591dba78700] main audio output error: module not functional
Aug 21 09:19:46 hostname vlc.desktop[2577]: [00007fb3a4094a70] main decoder error: failed to create audio output
Aug 21 09:19:47 hostname vlc.desktop[2577]: libva info: VA-API version 1.7.0
Aug 21 09:19:47 hostname vlc.desktop[2577]: libva info: Trying to open /usr/lib/x86_64-linux-gnu/dri/iHD_drv_video.so
Aug 21 09:19:47 hostname vlc.desktop[2577]: libva info: Found init function __vaDriverInit_1_7
Aug 21 09:19:47 hostname vlc.desktop[2577]: libva error: /usr/lib/x86_64-linux-gnu/dri/iHD_drv_video.so init failed
Aug 21 09:19:47 hostname vlc.desktop[2577]: libva info: va_openDriver() returns 1
Aug 21 09:19:47 hostname vlc.desktop[2577]: libva info: Trying to open /usr/lib/x86_64-linux-gnu/dri/i965_drv_video.so
Aug 21 09:19:47 hostname vlc.desktop[2577]: libva info: Found init function __vaDriverInit_1_6
Aug 21 09:19:47 hostname vlc.desktop[2577]: libva info: va_openDriver() returns 0
Aug 21 09:19:47 hostname vlc.desktop[2577]: [00007fb3a4094f10] avcodec decoder: Using Intel i965 driver for Intel(R) CherryView - 2.4.0 for hardware decoding
Aug 21 09:21:01 hostname CRON[4334]: pam_unix(cron:session): session opened for user root by (uid=0)
Aug 21 09:21:01 hostname CRON[4335]: (root) CMD (   test -x /etc/cron.daily/popularity-contest && /etc/cron.daily/popularity-contest --crond)
Aug 21 09:21:01 hostname CRON[4334]: pam_unix(cron:session): session closed for user root