1. First, open the AU software and import the audio file whose volume needs to be adjusted. Find the audio area on the AU track and select the part where you want to adjust the volume.

2. Secondly, in the control panel or mixer of AU, look for the volume control slider or parameters.

3. Then, adjust the volume slider or parameter to the range of minus 6 to minus 9.

4. Finally, export or save the adjusted audio file.