Perhaps something like mpegedit could do the job? It's open-source, and has an n-curses interface option for CLI. A QT interface would rock, but CLI would work if in a bind? It appears they have a python extension library as well, which could be good for long-term tinkering as well. If someone with the time would be willing to pick it up and compile it for the N900...

Edit: Also, depending on how you do it, there are lots of tools already available. sox, for example, is in extras-devel, which can use the trim command to trim things. Unfortunately, it's not compiled to use lame/mp3. But if you have a sample in wav format, or can get it there, you're good.