Music and sounds need to have the same names that are used in the games, for example if you want to replace music of MAP01 in Doom2, your MP3 has to be named D_RUNNIN.
As for sounds, they need to be on a new wad, if you want it to be loaded automatically, rename it to wad1.wad and place it the basev/game folder, where game is doom, doom2, heretic, hexen, etc.
Hope this helps you out <!-- s:) --><img src="{SMILIES_PATH}/icon_smile.gif" alt=":)" title="Smile" /><!-- s:) -->.