The OSD Audio ATM7 7-zone speaker selector dose not provide impedance matching, try one speaker distribution selector with impedance matching, if still has insufficient music volume then add a higher power stereo amp connect to the Onkyo front R/L pre-out to drive those speakers.
https://www.amazon.com/5-Zone-Stereo-Speaker-Distribution-Controller/dp/B01G7TW8W0
Onkyo R/L pre-out ---> stereo power amp ---> speaker selector with impedance matching ----> speakers
https://www.amazon.com/5-Zone-Stereo-Speaker-Distribution-Controller/dp/B01G7TW8W0
Onkyo R/L pre-out ---> stereo power amp ---> speaker selector with impedance matching ----> speakers