Mojave thunderbolt displayport sound not working

Hi,

I use an iMac as a monitor which is connected via mini displayport using a thunderbolt to thunderbolt cable. Since upgrading to Mojave I can no longer listen to the audio playing from my Macbook through the iMac's speakers; this worked fine before the upgrade. The iMac shows up as an output device, but switching to it results in silence. Does anyone have a solution to this issue please?