Adobe Audition CS5.5 provides a comprehensive set of audio tools which can handle just about any audio processing task.
If your source audio is poor quality, for instance, then Audition's versatile set of repair tools will remove pops, clicks, hums and more. The Adaptive Noise Reduction tool is smart enough to remove sounds which vary over time. And the Spectral View graphically displays your audio glitches, and allows you to visually clean them up with the Audio Healing Paintbrush.
If you simply want to enhance the current audio then Audition's built-in multiband compressor provides detailed control over dynamics, loudness and more. There are options to visually adjust gains, volume or fades, while a quality time stretching tool alters tempo without changing pitch. And improved workflow means you're able to tweak multiple settings simultaneously, and hear the results right away, so helping to deliver the best possible audio.
Other highlights include the batch processing options, which can convert the format of multiple files in a single operation. The Resource Central panel gives you easy access to thousands of royalty-free effects, loops and more, ready for use in your next project. There are excellent Guitar Suite effects, VST support allows you to extend the program's capabilities even further, there's full 5.1 multichannel support, and you get easy integration with Adobe Premiere Pro CS5.5, amongst many other essential features.
Note that the download here is for the Adobe Download Assistant. You'll need an Adobe login and account to download Audition.
It's not cheap, but that's no surprise - Audition CS5.5 is packed with essential features, and is one of the best audio production suites around