Either your voltage regulator has died or your battery is shot. I'd vote for the battery being overage -- they tend to lose their capacity and take too much current without actually charging. The low voltage radio dropout (lack of sound) and low rpm sounds a lot like a dead battery to me. When I bought my 300TE I had to jump it three times on the way home, wouldn't idle, the SRS and ABS lights stayed on, etc. New battery fixed it.
Were you headlights too bright? Overcharging actually means too much voltage -- you can check that with a standard cheap voltmeter -- just check the voltage across the battery with the engine on. Normal is 12.5 to 15 volts, any more and the regulator is shot.
These cars have a complicated voltage and current control system, not just a simple regulator like the old ones, and I've not played with that yet.
Any reputable mechanic and many autoparts stores can test this for you.
Don't buy a cheap battery.....!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!