Rogue Amoeba last week released, a simple audio-manipulation program which can handle AIFF, MP3, unprotected AAC (m4a), and Apple Lossless format files. Fission's most notable feature is lossless editing of AAC and MP3 files, a capability that's currently unique with regard to AAC and for MP3 found only in certain high-end editing programs or in the discontinued and now free . Freeverse's $80  uses third-party support for editing MP3 and AAC, and can import and export in those two compressed formats, but it can't provide lossless, native editing that avoids re-encoding when saved.
Like Rogue Amoeba's other products, Fission has focus - it's aimed at helping you, but doesn't offer all the features of other audio editors. In other words, there's a Delete item in the Edit menu but no Paste. You can remove audio by selecting it and clicking the Cut button at the top of the audio window. Choosing Cut & Split or using the Split tool divides an audio file into several clips, each of which is saved as a separate file when you're done with the file. Fission also replaces the Save command in the File menu with Save Audio (Command-Shift-S, like most Save As commands) to avoid overwriting the original file you're editing. Finally, a Crop tool removes everything in the file except the current selection.
The idea is that you can trim unwanted bits from audio you've captured or created, such as commercials in the middle of radio shows, "ums" and static from a podcast, or even the gratuitous applause at the end of a live recording. Fission can modify selected segments for fade-ins and fade-outs, too.
When you're working with an AAC or MP3 file, Fission makes changes directly to the original audio data, avoiding a cycle of decoding and re-encoding that would produce quality loss. The program can also export to any audio format available through QuickTime, including AIFF and WAV. Rogue Amoeba makes much of the fact that you can create ringtones for some cell phones by trimming and fading files, and then saving them as MP3 or AAC to phones that can import those audio formats for ringtone playback.
Fission's interface is delightful, offering the scrub approach to selection, in which dragging a playhead through the audio plays back whatever is at the playhead, skipping through it at the speed you're dragging. After making a selection, you can drag a playhead on the left to scrub backwards, or on the right to scrub forwards. (Scrubbing can be disabled from the Preferences dialog box, too.)
Fission costs $32 and is a universal binary; there's a in which saved audio is intentionally degraded with a series of audio fades. Owners of  can obtain . Fission requires Mac OS X 10.4 and is a 2.5 MB download.