You get a radio card at the Goldenrod radio tower by completing a five question quiz.
you have to get the radio card in kanto then go to it and go to the top and it will be a radio station called pokeflute
You can't do Pokemon contests in Pokemon soul silver.
on certain days there are songs for different regions to get mawile use the pokegear radio and use the hoenn song.
route 42
In the radio tower after you answer a quiz
you get it by ansering questeons right at goldenrod
Lavender Town> Radio Tower> Someone
use the stylust and go to the top star on the radio.
In Lavender Town, where the radio tower is, there is in old man inside and he will give you the Kanto Radio card (if it is your first visit)
go the radio tower in goldenrod and take a quiz at the counter and she will give you a radio card
Cinawood city. Inside the pokemart which is blue most likely located in a city near a Pokemon center.
get a radio card and play the poke flut song on you radio