I can save space by reducing bitrates of songs to x*, the value beyond which humans don't discern any difference. If so, how to reduce the bitrates with CLI:
1. mp3 to mp3 conversion
2. during mp3 to ogg conversion
3. ogg to ogg conversion
Of course, if the original bitrate is inferior than this "x", I don't want to artificially increase, I dare say.
* Reference to other question:
What is the maximum audio bitrate humans can distinguish?