Thanks for reaching out to the community about this!
At the moment there is not a direct way to pull the length of an MP3 file from the attachment field. I can see how this would be helpful to those adding MP3 files to their base. I’ve shared your vote for the idea with our team, and you can keep track of new product updates / feature releases at www.airtable.com/whatsnew.
A quick workaround is to format your file names to include the length of the MP3 and then use a LEFT(), RIGHT(), or MID() formula function to pull that information from the file.
It would look something like this:
I hope this helps! If it does, would you mind marking this post as solved? If not, let me know what other questions you have.