Hi,
No, it’s not possible to select the codec that is going to be used. The network provider is the one selecting the coded to be used.
The synchronization between the PCM samples and the codecs is fully controlled by the telecom stack, as it depends on the time when the samples will be sent towards the network (and that have to be encoded using the right codec). No possibility to change them by the user as it would disturb the behavior of the telecom part.
Thanks