摘要 (TL;DR)

Technically speaking, it is impossible to convert FLAC to MP3 without losing any quality. This is because FLAC is "Lossless" (like a ZIP file for audio), while MP3 is "Lossy" (it permanently throws away data to shrink the file size).

However, you can make the difference imperceptible to the human ear by using the right settings. Here is how to do it correctly.