Problem with USB speakers and new Mac Mini

I have a USB sound system consisting of a pair of Harman Kardon soundsticks and a woofer from probably 10 years ago (bought and used originally with a Mac). I have tried driving it my new Mac Mini. It works fine for about a minute; then the music stops. Here's what I've tried to diagnose the problem:

1. The problem does not occur when these speakers are driven from my Macbook Air (about 3 years old).

2. The OS and iTunes software is up to date on both machines (10.9.5 and 12.0.1.26).

3. It does not depend on which USB port I use on the Mac Mini.

4. Using a port on a USB extender on the Mac Mini does not solve the problem.

5. The iTunes Preference settings are the same on both machines.