You can get good results if using the oem microphone.
Microphone in black for Porsche 986 and 996 - Porsche Classic
If my memory serves me right, the Mic has an adjustment screw to set sensitivity for optimal performance.
Edit: another link for earlier models with better price.
Suncoast Porsche Parts & Accessories Dash Microphone
Edit again: the suncoast part is only the plastic trim, without the mic.
found out the hard way.
although the description on their website clearly states:
"Replacement dash trim. Small trim near the instrument cluster,
with microphone for the audio system.
Fits the following models:
- Boxster models 1997-2004
- Carrera/Turbo/GT3/GT2 models 1999-2004"