Editing an audio or video file is easier than you might think...

Trim audio

Not a lot of people know this but if you need to trim a video or an audio file, like an MP4 (video) or an MP3 (audio), it's easier than you think. Just open the file in Quicktime Player, and use the editing features to trim the track.

Audio trimming in Quicktime

Edited audio can be saved it to iTunes as AAC (Apple's version of an MP3) using the Export option, if you really need it to be an MP3, iTunes can convert audio files to MP3 if you need it to.

Now you can do more complex editing, but not as an audio file, so one way round this is to record the audio as a video, then edit as described below, then at the end discard the video and export as audio only. The same works in iMovie.

Edit video

If you're not bothered about transitions, and picture in picture, then the editing option in Quicktime are all you need. You can trim the video just as you a would audio, (which incidentally is exactly the same on any iOS device, same conventions). But where it really rocketh is in it's accuracy with splitting and deleting bits of video, or moving them around.

Just choose View > Show Clips to reveal a simple editing timeline at the bottom, click to select it (it will have a yellow outline) then move to the section of the video you want to split, you can even nudge the split point frame by frame using the arrow keys.

Once you've found the split point just choose Edit > Split Clip (or command Y) and you will see the video split into two (or more, if you split more than once) lozenges. Those bits can be deleted, or even moved about, simple.

You can even view the audio waveform to make editing easier, see where the quiet parts are...

Combine multiple clips

Just drag a video onto a video you have open and see the magic unfold before your very eyes! Now you can trim, move, split and of the clips. 

When you're finished Export and choose the kind of video you want.

Convert video/audio file formats

For a final phenomenal feature, got video that is to big? No problem just right/secondary click, choose Encode Selected Video Files. The in the dialog window that pops up choose a size, 480p will create a smaller, web friendly version of your video.*

* You can make video files smaller, but you cannot make them bigger!