All I want to do is add the length of the mp3 track to the filename [i.e., filename (5m 13s).mp3]. What is the easiest way to do so? I tried %_length% and other attributes but nothing worked. Please help!
Use this:
%_filename% '('$replace(%_length%,:,m )s')'
as a column heading.
Then apply the code as needed.
You can use Convert Tag -> Filename (ALT + 1)
with this Format string:
%_filename% '('$replace(%_length%,_,m )s')'

Or an action from type "Format Value" for the field _FILENAME with the Format string:
%_filename% '('$replace(%_length%,:,m )s')'
as @ryerman wrote.
Works perfectly! Thank you very much, LL!