YOU NEED TO FOLLOW THE INSTRUCTIONS CONTAINED WITHIN THE BOX AT A MINIMUM.
For best results use 3 or more wires per 5v connection and 3 or more wires per GND connection if you do not have either the large or baby breakout board.
You need a minimum of PSU 2.5A as per instructions, it needs good regulation and low ripple to get best results; ie a GOOD quality regulated power supply, that supplies the correct voltage and is rated at the correct current.
A current rating greater than 2.5A is ok.
A current rating LESS than 2.5A is NOT ok.
Buck converters are not recommended.
The 12v & 5v supplies need a common ground; ie ground connected together at some point.
If any of this is confusing or not clear please find someone that is experienced in these matters.
Simply put:
Supplying ONLY 5v means the board will boot, but reduced output will be available at the speaker connectors when speakers are connected.
Supplying 12v AND 5v means the board will boot and full output will be available at the speaker connectors when speakers are connected.
Supplying ONLY 12v means nothing will happen. No boot. No Sound.
Be aware that sound output is also dependant of the correct drivers and software configuration.
The empty space on the board that is marked power is for a button, you need to look at the schematics to see how this connected. DO NOT apply 5v or 12v or any power at this point.
YOU NEED TO FOLLOW THE INSTRUCTIONS CONTAINED WITHIN THE BOX AT A MINIMUM.
You may also want to visit this page as well.
https://www.reddit.com/r/Atomic_Pi/comments/bp0hvt/megathread_powering_your_atomic_pi_ask_all_your/
22/06/2019
Edit to correct statement regarding sound with 5v only.