After a drop my mic is not working properly.

In talking they hear me very low. Lets say 5%
If a turn on speaker while talking it goes to 20-50% depending
distance mouth - n900.

I tried alsamixer -c0 but dont know that mixer bar represents the mic.

As far as hardware inspection - fix , is that doable ?

If they can still hear you at all, then the mic is still working. There is probably just something blocking it mechanically. The first thing to try is open the phone and check if any dirt, protective covers or anything like that moved around in the impact.

Do i need special tools to open a n900 , like those mentioned in the service manuals?

Also in upper-left and down-left side of the cover there are a mesh of little holes. Are those for vantilation or and for sound passage? In the second case if they got cloged with dirt could that be one explanation of the low mic sound?

All you need is a Torx screwdriver. Can't remembered which one, T6 or T7. I used a universal one for £1 from a pound shop. And maybe a credit card or something similarly flat and non-scratchy to help holding things prized open.

A Torx T6 and a Philips Ph0 will do the job. Overall, do-able,
also the compiling together again