It would be nice if there was a keyboard shortcut to “Export as MP3”. Most others under the file menu have one, including “Export as Wave” and “Export as Multiple”.
That’s because Export as MP3 isn’t in Audacity. MP3 Export is only available after you intentionally modify Audacity with the addition of the lame software package.
If you don’t do through all that, there is no MP3 and the shortcut would do nothing.
Koz
In Audacity 1.3.x you just use Alt+F then E.
The export format will default to whatever you used last.