Hi Rajesh,
linphonerc_default are settings used to create the linphonerc settings at first linphone run
linphonerc_factory are settings overriding whatever is in the linphonerc_default file.
Example:
if there is a linphone_factory with:
[sip]
media_encryption_mandatory=1
Even if you set
media_encryption_mandatory=0
in your linphonerc_default, the media_encryption_mandatory setting will stick to 1.
I'm not sure of the exact mime names and sampling rates required, but you can use something like this to order the codecs:
[audio_codec_0]
mime=g729
rate=8000
enabled=1
[audio_codec_1]
mime=pcmu
rate=8000
enabled=1