To quickly process audio files from within the Finder, use Rapid Convert.
Select file(s)/folder(s) in the Finder and CONTROL+CLICK one of the selected items to bring up the contextual menu.
Select Rapid Convert (Show Settings) from the Sound Grinder section of the contextual menu (in the More menu in OS X 10.5 and later)
Choose your conversion options.
Click the Convert button to initiate the processing.
The Rapid Convert Progress window will appear. This window will
automatically close when the processing has completed.
Note: For faster processing, set your common Rapid Convert settings in Preferences or create new presets. Then when choosing Rapid Convert in the contextual menu, simply select Rapid Convert (Default Settings), and all specified files will be processed with those settings without any additional steps.
Note: For even faster processing, create a Droplet!