In audio (as opposed to tires) this refers to the process of replacing capacitors in equipment, usually a mixing board. Analog mixers employ hundreds or even thousands of capacitors throughout, and the quality of many of them can have a direct impact on the sound quality of the mixer. There are a number of different types and configuration of capacitors, some of which age better than others or otherwise have different characteristics that make them more or less desirable to use in a given design. In an older mixing board it is possible that some of its capacitors will begin to break down and become unstable. This can produce a wide variety of symptoms including phase shift, filtering, and distortion from DC leakage.
Aside from the obvious need to “recap” a mixer as a result of its capacitors beginning to fail, some audiophiles claim that certain types or brands of capacitors can make a huge difference in the audio quality of any mixer, and will therefore recap “perfectly good” mixers to achieve the desired results. This is a practice that dates all the way back to the first production mixers to be built and still continues to this day and is where the mystique of “recapping” a mixer originates. The idea being that it is hot-rodded or otherwise improved from stock designs can add big dollars to the sale price of such a mixer, or to the hourly rate of studio time.