I add more details in order to help to fix bugs related to "internet" (sip) calls :
- i use MIUI-2.1.20 on Samsung SGS2 (GT-I9100)
- integrated SIP client uses wrong (upper) microphone -> confirmed !
- if i edit an already created SIP account, server and protocol settings of that account are lost and i must insert them again.
Traditional (GSM) calls are working fine and correct microphone is selected.