Combining 2 audio outputs

I have an app that is play music to the default audio device (which uses pipewire). When I using a second play command (pw-play) the command executes but that audio is not heard. I was hoping to merge / combine the two audio streams. Is there a way to cause the audio to be combined? (Instead of the first play command only being heard) **UPDATE**: I did have problems getting sound to play (from a service running as root). So I created a script that runs pw-play using sudo, and that seemed to work on its own. When I run the script (from the service) I hear audio play. But, if I'm playing music (using shairport-sync) and then run the script it will not result in additional audio being played. Using 'ps' I discovered that a bunch of my playback scripts are in state code S, so are suspended and waiting on something. Even the speak-test I ran is in S state. I tried restarting the shairport-sync service but that did not allow the other processes to continue. I'm not sure how to diagnose this - how can I figure out what these stuck processes are waiting on? If pipewire is supposed to merge audio then it shouldn't block the scripts...