The line-in input on your Mac isn't meant for a microphone. Instead, you'll need a USB audio interface. Learn about them and what type of microphone is used to make MacMost Now.
You can use GarageBand as a simple audio file editor. Just drag and drop a sound file into GarageBand to edit it. You can adjust the volume, cut out parts of the sound and apply filters. You can then export as a standard sound file or a compressed file.
Since your Mac can handle more than one audio device, it is important to know how to tell it which device to use for output and input. For instance, you can have external speakers and a USB headset, as well as several microphones. You can set this system-wide, or for individual applications like iChat, GarageBand, Audacity and Skype.
Learn how to make a simple music loop using GarageBand in this excerpt from the MacMost.com Guide to Switching to the Mac by Gary Rosenzweig. You can use simple loops in photo slideshows, videos and other projects.
Many users never change the iTunes Import Settings, but it is important that you check them out and make sure that iTunes is importing your CDs using a format that you like. The default AAC may not provide the versatility that you need.
You can use GarageBand to edit your video's audio. You can filter the audio, add music, adjust the volume and re-arrange the sound.
Learn how to create layers of audio in iMovie 09 and to adjust the volume of each layer and add sound effects to your iMovie projects.