Instructions
Complete the Lavender Town portion of the main questline, as well as the Repair the Radio Station side quest. This will gain you access to the PokeFlute channel on your radio.
Return to where the Snorlax is blocking your path forward on Route 11.
Open your Pokegear and select the "Radio" option.
Select the "PokeFlute" channel near the very top of the globe.
Exit the Radio and talk to the Snorlax with the PokeFlute station playing. The big Pokemon will awaken and attack you.