The car will power stuff up on anticipation of you starting it.
This means the amplifier will be on the minute you open the door
Depending on the model year you have some of the chimes come through the amp/speakers so even if the radio is off the speakers
and amp are active.
The car can remain awake up a half hour after you leave it if its not locked . If you lock the car and move keyfob out of range
it should power down within about a minute and go into sleep mode.