Even if you don’t plan to record or mix audio yourself and just use third party audio files, you need an audio editor tool. You’ll always need to do things like trimming files, adjusting volume and tone, sometimes add a few effects or create loops. Like with all my tools, I prefer free or open source if possible, and when it comes to open source audio editing Audacity is a must-have.

Audacity is a multi-platform (Windows, MacOS and Linux) audio editor. It can record audio, import all sorts of audio formats, mix multiple audio tracks and apply all sorts of effects. It has a nice list of built-in effects like bass boost, echo, reverb and noise removal, but it also supports VST plugin effects, making the list of possible effects almost endless.
Audacity can also export audio in Ogg Vorbis format. This is a compressed audio format that has a few interesting advantages over MP3. First of all, and many people don’t know this, the MP3 algoritm is patented. Although I have never heard of anyone getting in trouble because of this, you can’t just use it in an application. Ogg Vorbis, on the other hand, is patent-free. Second, the same audio quality can be achieved with half the bitrate. For sound effects this may not make a huge difference but for music it can make the final size a lot smaller, wich is especially important if you’re writing a downloadable game.
The functionality of this tool is a bit limited compared to commercial software, but the advantage of this is that it’s incredibly easy to learn. It has almost no learning curve and you can begin using it right after installation. Besides, the functionality is more than enough for basic audio editing and in most cases, enough for all your needs.