You need to configure the Output Plug-in that is enabled and then select the correct Audio Device to be used by the Output Plug-in.

I also have this problem. I tried Lowlander's advice and see three plug-ins, but I have no idea which one to choose. Just rotate through them?
mzman, did you have success? I'm using the newly applied 4.1.18.1853. on Window 10 (Media Monkey Library shows in the Settings window for Bluetooth, but nothing happens).

You may need to set the correct Audio Device in the enabled Output Plug-in (Tools > Options > Output Plug-ins).

I have a new problem that just started today. I am running MM version 4.1.17.1840 and recently MM will not play anything through bluetooth. I have verified that MM is the only program that will not broadcast through my bluetooth system. The problem started yesterday and a reboot of the system fixed it but today the problem exists again. There have been no updates to Windows 10 since it started and the fact that I am only having this problem with MM and no other programs, leads me to believe it is directly related to MM.

Has anyone else experienced this and does anyone have a solution?