If you have a Bury System 8 Universal Cradle and the audio from the car kit is very low & muffled it is worth checking the volume settings on the actual cradle. A lot of people don’t realise on the left hand side of the cradle there are Volume UP / DOWN buttons.
You need to be connected to the cradle via bluetooth and actually making a call on the car kit (perhaps call your voicemail) and then repeatedly tap the button to increase / decrease the volume. Depending on your model of phone you may even get a bit extra volume by adjusting the volume of your phone too, again whilst on a phone call.
This is relevant for the Original / XL / XXL version which look similar to below: