Many audio apps hang in High Sierra after an upgrade from Sierra I'm seeing several apps hang:


iTunes does not load. 
Spotify and other audio apps do not play sound. 
Chrome and Safari hang when playing videos. 


All the apps worked in other versions of macOS but do not work in High Sierra. Force quitting the apps shows stack traces with AudioToolbox in the stacks. 

How to fix this?
Mine is caused by an audio unit. Removing it fixes the problem. User and system audio units are in the folders 


~/Library/Audio/Plug-Ins/Components/
/Library/Audio/Plug-Ins/Components/ (be careful when you change the system audio units)


Some other audio add-ons seem to cause similar problems according to this web page https://weblog.rogueamoeba.com/2017/09/29/fixing-an-important-high-sierra-incompatibility/