iMac Speakers not working in target display mode

Hi,

I have a new MacBook Pro and an older iMac (late 2013), both running Mojave 10.14.6.

I got Target Display Mode working. (The Apple instructions for connecting fail to indicate that the iMac has Thunderbolt 2 ports while the MacBook has Thunderbolt 3 ports, so in is necessary to connect the two with a Thunderbolt 2 cable and a Thunderbolt 2 to Thunderbolt 3 adapter.)

The problem is that the sound is not output to the iMac speakers. Using the iMac speakers is a choice in Sound Preferences (Name:iMac, Type:DisplayPort), but the sound does not play. Using the MacBook speakers is a second choice (Name:Internal Speakers, Type:Built-in), and that works. However, I would much rather listen to the sound on the iMac's speakers.

Target Display Mode is smart enough to set the output device to the iMac, which is actually the worst case given that the sound on the iMac is not working, because to get sound I have to manually change the sound output every time that I connect.

I see that other people have been having this problem.

Thanks