Can a pin be Neutral, not high or low? I'm trying to figure out a way to controll a DFplayer mini mp3 player with ADkeys.
Because I only have two or three files to play at a fixed volume I find it easier to just use the ADkeys. https://wiki.dfrobot.com/DFPlayer_Mini_SKU_DFR0299#target_4 



Lets say I want to use Segment 1 - Segment 3 (MP3 file 1 - MP3 file 3). I would then connect a resistor (or none in Segment 1) in between and to three pins on the Arduino.
But how can I make the pin neutral, so that it does not trigger everything at once?
As far as I can read the pins are grounded in Arduino if not "high".
What you are talking about in this instance is "Open Drain".  Normally, as you rightly note, with outputs you have either HIGH or LOW, which effectively connects the output pin to +5V or GND respectively.

Instead you are asking for Open Drain.  In this arrangement a pin is either "active", in which case it connects it to GND, or "inactive", in which case it is disconnected from (almost) anything.

Most 8-bit Arduino boards don't have Open Drain capability on their IO pins (some more advanced 32-bit boards do, though), but it is easy enough to emulate it in software.

You can emulate it using "Tri-State" logic.  That is logic where, as well as HIGH and LOW you have "High Impedance", or "HI-Z".  HI-Z is pretty much the same as "inactive" Open Drain. On the Arduino HI-Z is the same as INPUT.

So you can emulate it by:


To activate a pin set it to OUTPUT and LOW
To deactivate a pin set it to INPUT


For example, to "press" one of your buttons for 100ms you could:

pinMode(4, OUTPUT);
digitalWrite(4, LOW);
delay(100);
pinMode(4, INPUT);