With Blackhole installed, invoking dictation doesn't mute sound output on internal speakers or external headphones

[camner]

I have a multi-aggregate output device (Blackhole + internal speakers/external headphones) on my iMac (Catalina). If I route the output to the multi-aggregate device via Sound preferences, invoking dictation (Fn key twice) mutes the output (which is the default behavior with Blackhole not installed. But, of course one cannot control the output volume.

If I select internal speakers or external headphones in Sound preferences, the output volume CAN be adjusted, but invoking dictation is NOT mute the output sound.

Uninstalling Blackhole returns the iMac to default behavior, which mutes output sound upon invoking dictation with internal speakers or external headphones selected.

Is there a workaround?