Like another, my battery has gone flat while listening to the stereo/CD with the engine off. With a jump it started right away both times. I used a hygrometer and two cells seemed dead but when I went to a Firestone store, they told me the battery and the charging system was fine (the counterman said he could not sleep if he sold me a new battery when the old one was fine.)

The only thing I can think of is that the battery is too small for the load and discharges too quickly even under a light load. It is possible that the alternator is too small too since I've noticed the headlights dimming slightly at night when the car is idling with the heater/fan and stereo on (this is the same problem Honda had with some of their motorcycle when they were trying to shave weight, the bike would stall at signal lights when the rpms dropped to idle while the rider waited for the light to change. The next year Honda beefed up the alternator.)