I would say too low voltage; the light I think you see is the 'alternator doesn't charge' and as a cause system voltage drops and the radio chickens out by turning itself off and beeps. It's pretty difficult to tell what electronics do when they get too little juice, but the beeping will surely be linked to the light you saw briefly.
I had a similar thing with my previous car (205XS); alternator did not charge which caused my radio to turn on/off intermittently and even my revcounter dropped to 0RPM (while driving
). Was caused by worn brushes in the alternator (after 'only' 140k miles
) hence the battery wasn't charged and while system voltage dropped, electronic things just stopped working one by one.
You can check the voltage on the battery (or measure in the cigarette lighter plug) with engine off and on. Off should be around 12.4V (depending on battery condition/charge) and with engine running it should measure 13.6-14.4V.
Difficult thing with worn bushes; (my 205 experience talking here) the alternator can charge for a few minutes and then cut out again. It'll still push out enough voltage to keep the dash light off, but will actually not charge the battery. I noticed that after a few push-starts in the morning
next upgrade in that car (after new alternator/brushes) was a voltmeter