You really can't. Since they have no moving parts in the turbine housing like a variable geometry turbo you essentially get what you get.
Turbine size, geometry, exhaust housing selection, downpipe/exhaust size etc all play a part in it as does the OEM engine itself. Aftermarket cams/pistons etc also all slightly change the exhaust note.
Long story short, you'll never get it to whistle like it did with a VGT, not even close.