Soundtrack Pro is a music composing and audio editing application which is made by Apple, it has a collection of just over 5000 professional instrument loops and sound effects.
Multitrack mode is where the instrument loops and sound effects can be arranged in synchronisation with the video track, and where narration or other external audio sources can be recorded. There are both single and multi-take recording features. As with all of Apple's loop arranging software, the project's master key and tempo can be set, and all of the loops will automatically be played at those settings, regardless of the key and tempo at which they were originally recorded. At the same time, the key and tempo for the whole project can be varied over time with the master envelopes.
Audio editing mode
The audio editing mode is where individual clips can be edited. The waveform, as well as a frequcency spectrum, can be viewed for the clip. The audio can be analyzed for common audio problems such as clicks and pops, power line hum and clipped signals, and then any discovered problems can be fixed. Audio can be faded in and out, the amplitude can be altered for a selection, noise can be removed and silence, waveforms and ambient noise can be inserted. There is also a selection history tool that cycle between previous selections. After zooming in on the individual samples in the audio waveform, the pen tool can be used to edit the wave directly. There is a time stretch tool for changing the speed of a clip.