How far does an audio signal get?

Moin, I'm just going to start with an example that doesn't normally make sense, but it's just about the principle:

I have a laptop from which I send a signal to a laptop 100 meters away via a 3.5 mm jack cable, do I have to expect signal interference or the like on the long distance?

As long as you have no other sources of interference on the way then no.

With interference and the signal getting worse and quieter. The longer the cable, the higher the internal resistance.

Very much depends on the cable and the signal that is pressed in.