[Radiorail]

Try using the rotary volume knob when a TA is being made.

 

[Radiorail]

Have you disabled TAs / switched off the car and Nav unit then powered it all on and re-enabled TAs? That may do a reset. of TA Or you could carry out a unit reset - there is a pin hole button somewhere.

I'd still try winding volume up during a TA as if it's at minimum it may be disabling the TA programme mute function.