For a passive summing mixer is a circuit to combine two audio signals to one signal.
Passive summing works generally ok, both with unbalanced and balanced circuits if all connected circuits are same type (mixed balaced and unbalanced is mixed business if it works as wanted or not). When the devices connected to all inputs are powered on, things work typically very well. In some cases if you turn off some devices connected to one input, it can start to introduce distortion to sound.
I have used my DIY passive mixer circuits several times with event PA system and TV broadcasting setup.
For my passive summing boxes I have used resistance values from few hundred ohms (professional equipment balanced/unbalanced interfaces, potentially long lines and audio transformers on system) to 10 kohm (consumer equipment short distance to amplifier) depending on case.
Typical audio outputs have output impedance in around 20 ohms to 2 kilo-ohms range.
Balanced passive thing would be used to combine a couple of identical sources. Microphone or mixer.
Here is one of my DIY passive mixer circuit I have used for balanced signals from professional equipment (600 ohm or lower impedance).