The only way I found was turning the radio volume to zero while the sat nav wasn't giving any instructions as the volume levels are separate.
Alternatively, during an instruction turn the volume up and it will change the instruction volume without impacting the radio volume.
On mine (2021 MIB2 High I think - has no physical buttons, just touchscreen) there is a "Volume" setting. Within that you can pre-set the volume for the traffic warnings and navigation announcement, so far so good. Also in there is a setting for "Entertainment Fading" for Parking - and Navigation. So you can set the radio volume to lower whenever the parking system is active and also whenever a navigation announcement is made.
Checked in my car, its in Radio - Sounds - Volume and there you set the volume levels you want to listen to things at, and if you scroll down there are the settings to reduce media volume to less than the nav announcements.