My name's Hunter, I'm the dev for the Android app.
You're right, the storage location for the lesson mp3 files isn't adjustable at the moment.
However, the thought behind having the sub-toolbar which displays the folder path in the Settings > Manage downloads screen was to eventually allow for the user to edit that location. This hasn't been implemented yet simply due to the fact that we had additions which we felt were more pressing.
But, it's no doubt still on the to-do list. We've had a few people requesting this feature lately so it should be coming sooner rather than later.
I'm the meantime, I'm sorry this is cramping up your internal storage! I completely understand the need.
Thanks for jumping on the forum and taking the time to post this! The feedback really helps plan and prioritize.