Sudden volume increase

I'm running the following configuration:

LibreELEC 8.2.5
Kodi 17.6
Git: a9a7a20
CPU: AMD AG-6400K
Gigabyte GA-F2A88XN-WIFI motherboard with integrated sound card

Recently the audio, regardless of input source (OTA TV via tuner card in PC, recorded video, add-on channel) will suddenly have a big spike in volume. Sometimes this volume increase will hold steady and sometimes it pulses, alternating at about one second intervals between normal and extremely loud. Stopping and restarting the playback medium fixes it until the next time. Worked fine for ages with the same configuration. No pattern of occurrence that I can detect.

I have a Vizio sound bar connected directly to the PC via a Toslink cable.

Any thoughts on root cause?

Thanks!