electronic scheme "R1=2500M" how many ohm?
Just as it says. 2500 megohm or 2.5 Gohm. 

This is a fairly normal value in a professional capacitor microphone. 

The capsule will be a capacitor of around 15-30pF; sound waves in air move one of the plates to change its value.

Work out what resistance you need to make an R-C high pass filter with C=30pF and a flat response to 20Hz. 

(EDIT : C101 surprises me. If it's a 10dB pad to increase max SPL before overload, I could understand that, but I would have expected it to be switchable for greater sensitivity)