Audacity, the free audio editing program, makes it easy to alter audio files to fill a specific time slot.
This software does not come with a manual per se, though it has a good help system. The online support is excellent, and a number of YouTube videos are available to get you up to speed in a hurry.
Audacity has greatly simplified the editing and preparation of audio files for commercial spots. It's very easy to remove background noises, to edit out coughs and errors, and to combine multiple files into a final mix. The best feature, though, is Audacity's ability to stretch or shrink a file to precisely fit a time slot, such as 30 seconds, even if the original is quite a bit shorter or longer, without changing the pitch.
Audacity is easy to install, to set up, and to use, and offer many processing options such as the addition of sound effects, with more optional plugins appearing frequently. These, like Audacity itself, are easy to obtain, install and use.
Audacity's results in technical terms are on a par with all but the most costly audio editors, and may equal or exceed them in some of the most critical areas. In any case, the results, using any properly equipped computer, are broadcast quality. Ease of use and pricing propel Audacity to the top.