Comment 1 for bug 1962479

This happened due to the pasuspender call in the Mixxx starter.
For optimal latency, Mixxx needs exclusive access tho the Alsa audio device.
If this is occupied by another media player via Pulsaudio, Mixxx cannot be configured. That's the reason.
As workaround you can start Mixxx from the command line and bypass pasuspender.
Than you can for instance use jack/pipewire or the pulse Alsa device which works like you describe but with a latency penalty.

We have also this bug to fix the usability issue:
https://bugs.launchpad.net/mixxx/+bug/1414456