Thanks to everyone for their tips and suggestions. To respond to Brian's question, my dropouts are between 2 and 3 seconds. I have done a lot of experimenting in an effort to isolate the variables.
First, given prior experimenting and Brian's comment, I suspected that the problem might be related to either my USB cable or my USB DAC. So rather than connect my Mac Mini to my USB DAC (which is inside my integrated amp), I used a 3.5mm to 2-Male RCA Adapter Cable to connect the MM's headphone jack to an analog input on the amp. This meant that I was using the MM's internal DAC. I was still using MC to play the same music files. I played music this way for hours with no dropouts. (If it weren't for the drastically inferior sound quality of MM's internal DAC, I'd just keep this setup.)
Second, to eliminate MC as the cause of the problem, I played the same music files using my normal USB setup (MM's USB output to USB DAC), but I used iTunes instead of MC. The dropouts returned.
Taking the above two experiments together, I initially figured that the problem was either my USB cable or my USB DAC. I was going to buy a better quality USB cable to rule that as the cause out. Then, if that didn't work, I was going to see if the dropouts persisted with another USB DAC.
But then I saw DJLegba's post about audio glitches affecting 2018 Macs. The glitch is caused by Apple's T2 security chip, and it impacts some external USB 2.0 audio hardware. I am not sure if this glitch is what is causing my dropouts, but it certainly seems like it could be. I read everything I could find about the glitch on several forums. One workaround that seems to have worked for some people is to use a Thunderbolt 3 Dock (such as the CalDigit Thunderbolt 3 Mini Dock). I'm not exactly sure how the dock solves the problem. But I believe it bypasses Apple's internal USB, thereby bypassing the glitch.
I also read that Apple is aware of the problem and working on a fix. But I've learned not to hold my breath waiting for Apple fixes. But FWIW, the final post on one of the forums (posted 3/16/19) says that "the most recent macOS Mojave developer seed contains changes that improve the reliability of USB audio devices." So perhaps an OS fix is on the way and my problem will be solved. If this glitch is the cause of my dropouts, that is.
I know that it now seems likely that MC is not the cause of my dropouts. So I'm not sure how relevant my posts are to this forum. But I figured that it might be worth updating people on my situation in case they are experiencing similar audio glitches with their 2018 Macs.
I'd appreciate any thoughts/suggestions that people have. Thanks again to everyone.
Josh