Why is it safer to not short left and right audio jack outputs together?
Line-level outputs expect a high impedance load, so the output transistors typically don't have very strong current capabilities.
The whole point of stereo is that both channels are not always the same, so it's not inconceivable that, for instance, at some point the left channel is at -1V while the right channel is simultaneously at +1V.
Now the output is effectively shorted. In the worst case, this can fry your output transistors.
The difference doesn't have to be 2V like in this example, for excessive current to flow. In principle, any difference in output voltage between the left and right channel will create a shortcut condition.
Even if it doesn't damage the output transistors, this will affect the sound quality, and probably not for the better. If you mix the channels together with resistors, the resistors will effectively create a voltage divider, and the signals will be mixed in a controlled (or at least predictable) way. Without resistors, whatever output transistor is &quot;strongest&quot; will &quot;win&quot;, and the signal will be mixed in an arbitrarily lopsided manner.