So I went ahead and deleted/reinstalled Roon on my iMac–keeping a copy of the backup in case it didn’t go well. It reinstalled with no problem. Then I deleted/reinstalled Roon on my MB Pro. The same problem persisted–could not get the MBP to play music from its own speakers. I found another user’s suggestion and tried to first set Roon up on my Apple TV. I followed his instructions (turning on the password for Apple TV/Airplay). I think I got it to work on Apple TV. Then I tried following those instructions on my MBP. I was able to enter the same password in there, but when I tried to play an album (I always test things out with Bowie’s Low), it wouldn’t play. I’d push the play button and it would immediately stop. So I have no clue what I’m doing wrong now, and I am thinking of just giving up on using Roon with the MBP.