Hide an adapter in ceiling
Don't put this DC PSU inside the ceiling.  These kinds of adapters are not 100% reliable.  Sometimes they get hot.  Sometimes they fail.  Sometimes the connector for the AC supply fails.
Find a suitable serviceable location for the PSU, near an existing or new power socket where it can be plugged in.   You can run the DC cord into a wall and to the first loudspeaker.
If the location you want the PSU is too far from the speaker for its existing DC cable, my suggestion would be to cut that cable in half, and splice in a length of 16/4 individually shielded pairs (two pairs).  Use one pair to extend the power, and the other pair could be used for the speaker's Aux In connection should you wish to use it in future.  The shielding may not be necessary but it's cheap and might help reduce hum and noise.  Ground the shielding at the PSU end.