"Space and time are not conditions in which we live; they are simply modes in which we think." - Einstein
In everyday life, we often refer to distance in "semantic" units. When giving directions, for example, most people would say, "take a right at the next traffic light," rather than, "walk 356 metres straight, then turn right." Similarly, when talking about time, it makes sense to refer to semantically meaningful units, such as beats in music or words in speech.
Unfortunately, the time model exposed by current multimedia frameworks is very much like giving directions using distances measured in inches and metres. Digital video is typically referenced by their frames, and digital audio by their samples. The mapping between these artificial definitions of time and more semantic units of time can be non-trivial, especially when manipulation of the media timeline is involved.