Right after you get the EXPN Card from the old guy in the Lavender Town, go to Vermillion City. Then run to the Snorlax on Route 11. Go to your PokeGear then go to the Radio part. Make sure the little circle thing is at the middle of the very top of circle, and then you will hear the PokeFlute sound. Then exit the PokeGear and talk to the Snorlax. You will go into battle with it. It is at a whopping level 50. This will be your only chance to get Snorlax. Hope I helped! Source: Experience
Note: You can only use the Poke Flute in Kanto
The item Poké Flute cannot be obtained in SoulSilver. However, there is a function similar to it. If the player restores power to the Power Plant in Kanto, the manager of the Radio Tower in Lavender City will give the player an EXPN Card, which allows the player to tune in on the Poké Flute channel on their PokéGear. When played, this channel has the ability to wake the sleeping Snorlax that is blocking the entrance to Diglett's Cave.
I was too looking for it everywhere, but its on the radio and you just need to go completly in the top left corner and the poke flute will play :). You can only get it after the kanto station manager in the radio tower gives it to you.
Hope I helped! :D
Source: exprience
The Pokeflute is a radio station program accessible on the Pokegear, once the expn card is obtained from the Kanto radio station manger in the radio tower in Kanto it can be used.