Fission can crop and trim audio, paste in or join files, or just rapidly split one long file into many. It's streamlined for fast editing. Plus, it works without the quality loss caused by other editors, so you can get perfect quality audio even when editing MP3 and AAC files.
If you need to convert formats, Fission can do that too! You can rapidly export or batch convert files to the MP3, AAC, Apple Lossless, FLAC, AIFF, and WAV formats.
Note: Now requires OS X 10.12 or later