The answer is: when you need to run your car, say experts in car audio.
A study by an industry body, the International Council of Sound and Image, found that sound machines were needed in almost a third of the cars sold worldwide in 2014.
A second study by the same body found that in 2014, more than 60 percent of cars were running a sound system, but by 2019, that number had declined to just a quarter.
That has been a trend, with the numbers of cars with sound systems declining since 2012.
According to the research, in 2014 a total of 6.7 million vehicles were running sound systems.
That number increased to 7.5 million in 2019.
But in 2019, just 2.2 million cars were equipped with sound machines, down from 2.7 in 2019 and a record low.
Experts also found that the market share of cars that have sound systems has been falling for the past few years, down to just 8 percent in 2019 from a high of 17 percent in 2015.
“The industry needs to invest in the sound technology in vehicles to maintain the market position,” said Ramesh Jain, senior research analyst at ICSI.
“When the industry does that, there is a strong incentive for manufacturers to invest,” he added.