OBS virtualcam process video but not audio

[Mazandaran00]

On OBS 26.1.1 (Windows 10) virtualcam (1 in number set) works fine on video, but audio from local multimedia (.mkv file) is missing. Virtualcam line is present in the Audio Mixer, but no signal level indication and no sound in Monitoring. However audio is present on recording, which is no help when operating a Zoom session.
Audio from other local sources (microphone and desktop), going through Virtualcam, are working fine.
My little project is hanging. Please help.