Audio Router is software that gives you the possibility of routing audio from individual applications to specific devices, like speakers or headphones. It is a free and open source sound routing application that's straightforward.
Here are some screenshots to illustrate the features and user interface of this application:
Route sound from various apps to specific audio devices. Screenshot of Audio Router - 1473px · 1027pxOpen-source project that is no longer actively maintained but still seems to do the job. Screenshot of Audio Router - 1655px · 1175pxRoute audio from each application to different output devices. Screenshot of Audio Router - 1772px · 925px