Audacity is an open-source tool for editing audio files. You can use it import MP3, OGG, WAV and MIDI files, mix them, add simple effects, remove silent portions or background noise. Audacity is also great for recording audio files, like a podcast. It's easy to use: you can cut and paste selections.
Audacity works on Windows, Linux and Mac. If you still use Sound Recorder or if you think you need to use a commercial audio editor, try Audacity.