How do you get pokeflute in Pokemon HeartGold?

First you must complete the machine parts quest before heading over to the radio station in lavender town. Inside, talk to the gentleman against the wall and he will upgrade your pokegear radio to be Kanto compatible. Once this is done go to the snorlax and open the pokegear radio. Manually tune the radio to the top of the tuner and the Pokeflute channel will operate. Then talk to snorlax and it will wake up and battle you.
after you get the power plant up and running go to the lavender town radio tower.
You can't get one. However, if you have solved the crisis at the Power Plant and received the EXPN card at the Lavender Radio Tower, you can move the blinking dot on the radio to the top star and it will play Pokeflute music in Kanto only.
There are many kinds of pokeflute in heartgold. But the original one used to wake snorlax is the one in the radio, use the ds stylus to move the radio signal to the top then you will hear a music... after that talk to snorlax and WALA HE IS AWAKE...or you want it for your good theme
if u have the up-grade its easy

make the first Pokemon in your set to hold the up-grade

then go to your pokegear, go to the radio, move the station all the way to the top

with your stylus and there's the pokeflute