Hi @Cigogne
Welcome to the Sonos Community!
Inverting your Era 300 should not disable voice control - I just tested with my own Era 300 and had no issues while it was upside-down. Are you sure you have not disabled the microphone either with the touch-control on the top of the speaker or with the physical switch at the back of the speaker? Disabling via either method will result in the microphone light going off - if the light is not on, the microphone is inactive.
If the light is on and voice control still is not working, I recommend you get in touch with our technical support team who have tools at their disposal that will allow them to give you advice specific to your Sonos system and what it reports.
One more point - inverted mounting is predominantly for wall-bracket mounting. Placing your Era 300 upside-down while on a shelf is going to adversely affect the audio reproduction, and is therefore not recommended.
I hope this helps.
"Common sense is the collection of prejudices acquired by age eighteen." - Albert Einstein