It sounds like the JRiver WDM driver might be the default audio device in Windows, which would result in it frequently being opened by the OS itself, and thus showing up in MC.
Thanks for the quick response Hendrik.
Indeed it was, and selecting the DAC as the default device fixed the problem.
One peculiarity, though. On this PC, WDM is displayed in Windows Sound Settings as "JRiver Media Center 23" (I'm a long term customer from way back when, but am running MC 31).
On another PC, I have WDM selected as default audio device in Windows sound settings but instead is displayed in Windows Sound Settings as "JRiver Media Center" (no version number). There is no similar WDM issue on this PC with IPC being initiated/displayed when no other app (e.g., Amazon Music, a web browser) is running.
I'm wondering if the different display in Windows Sound settings means Windows is loading/seeing something different with WDM selected on the respective PCs, and whether that's the source of the different behavior, and if so, any suggestions.
POSTSCRIPT: I manually uninstalled all versions of WDM prior to MC 29 (including MC 23). After uninstalling/reinstalling WDM in MC 31, and restarting the computer, the issue with IPC running when no other Windows app is active in Task Manager appears to be resolved, even when the default Windows audio device is set to JRiver WDM in Windows Sound Settings (i.e., instead of the DAC, MC 31 audio selects the DAC as audio output device). However, even though MC 23 is uninstalled, Windows Sound Settings still display "JRiver Media Center 23" as the only selectable JRiver WDM audio device option.