Unfortunately Voice Match (aka Ok Google) doesn't work in any mi 9 rom due missing feature in chinese rom version, base for any other xiaomi.eu rom
Here more details by mr. Eisberg a Xiaomi.eu mod developer:
OK google was working fine on my Mi9 chinese version with the chinese rom, then also ok on my global rom when I flashed it over. Now on EU rom and it won't work. Will it work with future updates for EU rom?