Hi,
If my understanding is correct you are planning to create a IVRS system which is stand alone and datas given by the customer should be sent to computer.
My suggestions are
1.As you already know you csn DTMF decoder to get user data.
2.For Audio output you can interface SD card to the arduino which consisits of all your Sound files in WAV format so that you can play corresponding to the user key press audio by choosing from SD card. which can be given to the input(i.e MIC of the GSM)
3. My last suggestion is to use hardware serial for the communication GSM and Arduino and soft serial for the communication between the PC and arduino.