Instructions
Defeat each member of the Elite Four in battle and then go to Olivine City. Once you arrive, Professor Oak upgrades your equipment to the National Dex. This lets you hear new music on the radio.
Play the game on a Wednesday. If you do not want to wait, change the Nintendo DS date so the device "thinks" it is a Wednesday.
Go to a location where the "Hoenn Sound" attracts Pokemon, such as the Bell Tower or Union Cave.
Tap the "Radio" icon in your PokeGear. Tap "Pokemon Music Channel." The "Hoenn Sound" music plays.
Walk around in a grassy spot to attract Pokemon with the "Hoenn Sound."