15. At least one computing device programmed to carry out a method, the method comprising: playing at least a portion of a media presentation at a computing device, the media presentation being defined at least in part by a master file comprising annotation data indicating a time when an existing annotation is to be played during playback of the media presentation, the computing device comprising one or more audio input devices; monitoring the one or more audio input devices for voice input during the playing; and upon detecting voice input and without manual interaction with the computing device: determining that the voice input comprises a new annotation to be added to the media presentation, wherein determining that the voice input comprises a new annotation comprising determining that a user is speaking in the direction of the computing device, wherein the determining that the user is speaking in the direction of the computing device is based at least in part on a signal strength of the voice input received at the one or more audio input devices being above a first threshold value and variations in the signal strength of the voice input received at the one or more audio input devices being below a second threshold value; determining that the new annotation overlaps the existing annotation by an overlap amount; recording the new annotation during the playing; modifying the annotation data to remove the existing annotation from the media presentation if the overlap amount exceeds an overlap threshold, and modifying the annotation data to shift the time at which the existing annotation begins playing during playback of the media presentation so that the existing annotation no longer overlaps with the new annotation if the overlap amount is less than the overlap threshold; adding the new annotation to the media presentation, the adding comprising adding annotation data to the master file indicating the time at which the new annotation is to start playing during playback of the media presentation and a reference to an annotation file storing the new annotation; and after the adding the new annotation to the media presentation, playing the media presentation, the playing comprising playing the new annotation.
15. At least one computing device programmed to carry out a method, the method comprising: playing at least a portion of a media presentation at a computing device, the media presentation being defined at least in part by a master file comprising annotation data indicating a time when an existing annotation is to be played during playback of the media presentation, the computing device comprising one or more audio input devices; monitoring the one or more audio input devices for voice input during the playing; and upon detecting voice input and without manual interaction with the computing device: determining that the voice input comprises a new annotation to be added to the media presentation, wherein determining that the voice input comprises a new annotation comprising determining that a user is speaking in the direction of the computing device, wherein the determining that the user is speaking in the direction of the computing device is based at least in part on a signal strength of the voice input received at the one or more audio input devices being above a first threshold value and variations in the signal strength of the voice input received at the one or more audio input devices being below a second threshold value; determining that the new annotation overlaps the existing annotation by an overlap amount; recording the new annotation during the playing; modifying the annotation data to remove the existing annotation from the media presentation if the overlap amount exceeds an overlap threshold, and modifying the annotation data to shift the time at which the existing annotation begins playing during playback of the media presentation so that the existing annotation no longer overlaps with the new annotation if the overlap amount is less than the overlap threshold; adding the new annotation to the media presentation, the adding comprising adding annotation data to the master file indicating the time at which the new annotation is to start playing during playback of the media presentation and a reference to an annotation file storing the new annotation; and after the adding the new annotation to the media presentation, playing the media presentation, the playing comprising playing the new annotation. 16. The at least one computing device of claim 15 , the modifying the annotation data to shift the time at which the existing annotation begins playing during playback of the media presentation comprising: splitting the existing annotation into a first annotation segment and a second annotation segment; shifting the first annotation segment forward in time to avoid overlapping the new annotation; and shifting the second annotation segment back in time to avoid overlapping with the new annotation.