Hey,
I am currently working on a podcast I want to run. Because I wanted to use Kirby for the site, I started writing a Podcasting-Plugin.
Currently this plugin offers an extended RSS-feed including all fields you need, to get your Podcast to the iTunes-Directory. It scans for MP3-Files and parses their duration and size. I am thinking about more features, like implementing the Podlove-Player.
Have a look: https://www.maurice-renck.de/x/3j297c
Moin.
I created something similar recently 
I had a slightly different approach. Each blogpost with a podcast template would be added to the feed. Audio files that are attached to the post will be added automatically.
If you want to add the audio length to a feed, you may use this in the xml tag:
<enclosure url="<?php echo $item->audio()->first()->url() ?>" length="<?php echo $item->audio()->first()->size() ?>" type="<?php echo $item->audio()->first()->mime() ?>" />
worked for me. 
Moin Andi,
ah, I didn’t realize that there are functions like size() for audio, too! Thanks for that hint 
Maybe I can get rid of the getID3-Lib that way.
I’ll watch your plugin on GitHub. Maybe we can exchange some ideas from time to time.