Hi,
The Quectel M10 module has two different audio ports. The GSM Shield is set up to used by default the first one, but the jack you can find on the board is attached to the second.
One way we found to make it work, was using the following AT command: AT+QAUDCH=1
You can find details on the AT command on the following datasheet:
The other problem we are having is the command is not saved on the board, and everytime the power is lost the command goes back to the default.