With loopback you can loop the sound of a program that is running on your computer in addition to doozzoo.
(Loopback is a software of the company "Amoeba", which can be used free of charge for testing with time limit.
The paid version has no time limit. To the manufacturer page)
Here is an example with Spotify. (Instead of Spotify, you can also hear the sound of e.g. Finale. Sibelius, YouTube).
1. open the program whose sound you want to transfer. (here: Spotify)
2. open Loopback and set up a new Auciodevice and create and connect the channels like in the screenshot:
4. select the "Loopback" device in the system settings under sound as input - here it is simply called "Loopback" exactly as you have named it in the loopback program itself.
5. Select the Device that you want to use to output the sound: here the headphone output.