what can I do to add a little bit more sound to that 500 sl 1991. I don't really want performance, cause I think it have enough but I want a little bit more sound from that car.

get rid of the center and rear muffler, add sport muffler.If you want more, get rid of the catalyc converters too and add resonators....
this baby will roar like a lion....i loved the open-header sound of my fathers S500 m119....

I have 3 vidio sound files of an SL600

1) Rear muffler removed.
2) Rear and center removed
3) Open headers

They all sound very good. I susspect and SL500 will sound about 2/3rds as nice.

John Roncallo