Audio dropout is a brief reduction or loss of signal that occurs during playback, and it can be found in both analog and digital audio tape sources. Read about digital dropout here. Dropout is the result of a defect in the carrier or playback mechanism. Analog audio dropout is caused by anything that diminishes the uniformity of the magnetic media, such as dust, tape damage, or a defect in the tape stock.
Can it be fixed?
Dropouts can often be reduced, but they can be difficult to eliminate during playback. If you get a recording with dropout, your best bet is to clean the playback heads and re-digitize. Should dropouts persist, production or access copies of files can be remediated using software tools (for example, Izotope Spectral Repair) that use digital signal processing (DSP) techniques to resynthesize gaps in audio content.
Visualization of an audio dropout from 1/4” open reel audio. It can be seen as the visual spike around 9 seconds in.