Use Audacity, but remember that with audio degradation, the loop points may not work very well as you my get clicking and popping sounds coming from the old ones. It's a common issue unless you know for a fact that you are encoding at the same bitrate or at a bitrate that won't affect the loop points. I always encode at the highest (meaning bigger sizes) to ensure quality is kept as much as possible, but for mobile exports, that wouldn't work too well.