With the video (or audio) file open in QuickTime, click Edit > Trim. You can also open an audio file in quicktime - Command-click or right-click it, point to Open With, and select QuickTime. QuickTime is the default video player, so you should just be able to double-click your video file.
mp4 file or another video with QuickTime. RELATED: Use Your Mac's Preview App to Merge, Split, Mark Up, and Sign PDFs