My guess is that your version of mediastreamer was not built with gsm support. You can have a look at the file 'configure.ac
' in the mediastreamer2 directory to see how that works. Most notably, the package 'libgsm1-dev' needs to be installed when mediastreamer is built.
If Mint 8 works with Ubuntu PPAs, you could try to upgrade to linphone version 3.3.2 with
The package in the PPA has gsm enabled (as long as the shared library 'libgsm1' is also installed).
Perhaps that works for you.