Mint's audio panel might not let you do it, but install pavucontrol, which is pulseaudio volume control, which can (I always install and use this only).  

When you play the video with whatever generating audio, pavucontrol should show in "playback" menu, with the destination audio as hdmi output card.  Change that to be your other audio destination, the usb dac, and that should be all you need to divert audio there.  If you don't see another device, ensure all are enabled in Configuration and you see them both under Output Devices.

Check and make sure all the playback applications are seeing audio levels, and then make sure the appropriate output device is as well under output does too as they can work independently.

-mb

On Thu, Feb 7, 2019 at 10:49 PM Victor Odhner <vodhner@cox.net> wrote:
My Mac Mini's Preferences > Sound window lets me “select a device for sound output.” This works well driving my sound system. It detects other devices but lets me select my converter:
    USB > Digital-to-Analog converter > mixer board.

Everything I find online seems to indicate that nobody’s created a similar configuration tool for Linux, that we have to use some queries and build a config file. The query I tried did show that there are two devices, but I really don’t know what I’m doing.

Any instruction would be greatly appreciated.

My situation:

In my Mint system, I want to drive a monitor via HDMI,
and sound via USB > Digital-to-Analog converter > mixer board.
The very same connection worked when driven by the Mac Mini.
Linux never had a problem with it either, but I messed up something.

At one point the sound was playing through the monitor’s relatively weak loudspeaker.

After some tinkering, nothing is coming through.

Thanks,
Victor

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