You’ve connected the speaker output of the APR9600 directly to the MIC input of SIM300. There could be impedance mismatch problems since the speaker output is normally used to drive 8 ohm loudspeaker. Also the speaker output gives signals much stronger than a normal microphone and when the built in amplifier of SIM300 amplifies such high amplitude signals, high signal distortion can happen.
Anyway suggest you to try out the result.
Waiting for your update!