macbook pro losing audio

I started participating in Seti@Home many years ago, and used BOINC Manager to participate in Einstein@Home, Rosetta@home, SiDock@home, and LHC@home. In the troubleshooting process of this sound issue, I started eliminating things that ran at startup, including BOINC Manager. Sound worked normally after this, and continued to work normally for a couple of days, so I started trying to determine what was causing the sound issue. I observed that I would lose sound within minutes of manually starting BOINC Manager (which starts whatever projects that are in the queue). I should add that the current versions of these projects require using Oracle's VirtualBox (free version). Note that normal sound function did not resume after stopping the projects and quitting BOINC Manager and VirtualBox. Each time it required restarting the computer. I do not know which project is causing the problem, so I removed all of the @home projects. I uninstalled BOINC Manager and VirtualBox. In the future, I may reinstall these applications and try to determine if there is one particular project that is at issue, or if BOINC Manager or VirtualBox is the issue.

Resetting NVRAM had no effect. If I log off and log on again, or restart, the sound appears to work again. Sometimes it works for several days, other times for only a few hours. As of right now, I have not rebooted or logged off/on since about 6:30 am today. All aspects of sound are working EXCEPT the alert sound selected under the "Sound Effects" tab of the Sound Preferences control panel.

I've been a Mac user for some time, so I can usually troubleshoot issues to resolve these types of problems. This one has me stumped.

As I said, all aspects of sound are working except for the sound selected under the "Sound Effects" tab of the Sound Preferences control panel. It is not an issue of speaker selection. I do not have external speakers or headphones attached, nor do I have Bluetooth speakers connected. "MacBook Pro Speakers" is the only option. Under the Output tab, clicking on the Balance or Output volume sliders results in the feedback sound being heard through the speakers, but under the Sound Effects tab, neither selecting an alert sound nor clicking on the Alert volume slider will result in no sound.