To wake up the Snorlax on Route 11 in Pokemon SoulSilver version, the player has to open his Pokegear and tune it to the Poke Flute station. After waking up the Pokemon Snorlax, the player should talk to it to start the battle. The item Poke Flute itself can be found in Pokemon Emerald, FireRed/LeafGreen and X/Y versions.
The following shows how the player can obtain the Pokegear to wake up the Snorlax in Pokemon SoulSilver.
After repairing the Power Plant, the player must return to the Radio Tower in Lavender Town. He will receive an expansion card for his Pokegear from the suited man as reward for fixing the tower. This will allow the player to tune in to the radio channels of the Kanto, which includes the Poke Flute station.
The player must purchase some Ultra Balls at the Pokemart and then proceed to Route 11. While on the way, the player will encounter repeat Pokemon, items and TMS like Hypno, TM86 and Revive. Four trainers will also be on Route 11.
The Pokemon Snorlax will be on the left of Route 11. Once the player gets to the Pokemon, he should take out his Pokegear and tune it to the Poke Flute station. Once the Snorlax has awakened, the player must then speak to it to start the battle.