MP4 files with 320bit rate audio

Hello,

Am quite new to editing video!. I am editing music videos for my DJ shows.

I am trying to export as a MPEG4 640x360 with 320kbs on the Audio,

Once changed all custom settings it keeps exporting at 128,

Compare this size to a pre made music video I am missing about 100mb and the quality is not there.

What can I do to change this and get the correct setting I need