How do I use Spotlight to find all MP3 that are encoded at 320 kbps?
Open up Finder and use the Spotlight/Search on the top right of the window.
Type .mp3 in the search bar and Finder will suggest you things like Filenames (Name matches: .mp3), Kinds (MP3 Audio), Downloaded from (...)
Select MP3 Audio type under Kinds section in the suggestion popup.
There is a secondary toolbar which appears below the main Finder toolbar and above the search results, which has options like Search (This Mac) (User home) (Shared)  Save button  (on right side) and + icon
Click on the Plus icon in the right most corner of this toolbar. This will give you a list of filters/attributes that you can apply to current search. For eg. Kind, filesize, created date etc
From the filter type menu choose "Other..." and enable "Audio bit rate" attribute.
Now select the newly enabled "Audio bit rate" attribute with equals condition and enter 320000 as the value.


You will see a list of files which are encoded with 320kbps bit rate.

To see files with 128kbps change the value to 128000