Copy all the Mp3 song which you want to play in Voice City cars or Bikes etc and then goto
C:\Grand Theft Auto - Vice City\mp3.

If you can't access the mp3 folder then open C:/ and click on
Tools >> Folder options, in View tab click on "Show hidden files and folders" and click Ok.

Now open "Grand Theft Auto - Vice City" folder where you will see MP3 folder.
Open that Mp3 folder and past the Mp3 music that you have copied.

Now open Voice city Game and get into car, press R again and again so that Mp3 Player start.
Enjoy your favourite music.