peter I will see, if I can disable compression (does not make any sense with audiofiles anyway)
That does depend on what audio file format you are using.
MP3s I would agree. You'll probably only get 5% compression on average.
WAV files will comfortably give you 20% compression on average and can vary wildly depending on the content.
As someone who was using all WAV files (9GB) I'd happily take a 20% size reduction on the backup files! 🙂
I can appreciate that any compression library is going to need close to the same amount of free space as the content (give or take) in working space to do it's thing and that could get very unweildy on iPads where space can often be at a premium.
Dare I say it, an optional toggle (off by default) in Backup dialog to enable compression? 😛
But seriously, I have seen similar in other apps. There could be a label with the toggle that says something along the lines of "use compression for potentially smaller backup file, but more free space needed to create backup. It if fails, turn compression off".
If not inappropriate to ask, which zip library are you using? Have you looked at https://7-zip.org/sdk.html (public domain)?