either gauge will work for 12 meters. 12 ga will cost you less. 10ga might be too thick for your speaker terminals. Better check your speakers first.
Which cables need the Atmos + Danon 8500 speakers
6 speakers Atmos 6 meters to 12 meters
12 AWG or 10 AWG ?
https://www.bluejeanscable.com/store/speaker/index.htm