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To add to the audio compression: it isn’t possible to further compress an mp3 file without losing any quality. You can either:
- Recompress to a lossy codec (mp3, aac, opus). This will lead to smaller file sizes if you set the bitrate lower than that of the input file, but it will always worsen the quality, no matter the bitrate.
- Recompress to a lossless format (flac easily being the best one). Going from a lossy to a lossless format will increase the file size (sometimes by quite a substantial amount), while keeping the same quality. There is very little reason for you to do this
- keep the original files (my recommendation)
If you’re willing to spend some extra time learning about audio compression, you can download lossless files and compress those directly to whatever format and bitrate you want. The quality will be better than option 1 above, as the audio is only lossely compressed once instead of twice.
It’s possible for a certain hardware/software setup not to support a certain codec. For example, my jellyfin client (Finamp) uses the iOS native decoders (afaik), which means opus files are practically broken. My music library (8000+ songs) contained exactly 1 lossy file, which just so happened to be an opus file. I decided to spend the extra ~20MB to standardise my entire library to flac files, ensuring I could play every song on all my devices.
Edit cause I posted too soon: you are generally correct; only in very specific circumstances will you encounter compatibility issues like this one in the modern world. This is 100% apple being apple, and you can expect pretty much every other (reasonably modern) device to support all codecs you might encounter in the wild.