Ensure that your soundcard is running at the same sample rate as the audio in the timeline. When working with video, audio should be 48kHz, most soundcards will default to 44.1kHz, meaning your computer has to do some resampling on the fly. If you’re resampling and doing some heavy video playback at the same time your laptop could falter. Go to Preferences>Audio Hardware – check the sample rate then click on Settings to ensure your soundcard is set to the same setting.