As far as I can remember, the problems started after I dropped the device.
The other party wasn't hearing me during calls and also Voice Memos intermittently just recorded static/noise.
The international shipment of the parts was very quick. Ordered on Wednesday and received all soon after noon at Friday.
The guide (and some Youtube videos) really helped me replace the dock connector.
The problem seems to be fixed. I'm no longer having problems during calls or static in voice memos.
This or similar mic problems may sometimes have something to do with an integrated audio chip that has a very advanced repair procedure as far as I can see on the net.
So try to replace the dock/mic assembly first if you are not 100% sure what is causing the mic problems...
That's all, and thanks iFixit.