Send SMS when music stops

Hello to everyone, I register in this forum because need a little help.

Well, I work in a radio, and the software what we use have several problems, so my idea is make a program that notifies me when the music of the radio stops.

I will buy an GSM (SIM900) module, Arduino Mega 2560, but I don't know which audio sensor can i use. Or directly connect the Line Out of the PC into the Analog input of Arduino, using a 10K resistance and a electrolytic capacitor.

I'm wating your answers, thanks a lot.

Use one of these

You can replace the microphone with a direct connection to your audio from the radio. If this audio signal is too big then cut it down with a potential divider.

Grumpy_Mike:
Use one of these
Electret Microphone Amplifier - MAX9814 with Auto Gain Control : ID 1713 : $7.95 : Adafruit Industries, Unique & fun DIY electronics and kits

You can replace the microphone with a direct connection to your audio from the radio. If this audio signal is too big then cut it down with a potential divider.

Thank you Grumpy_Mike, I'll use it!!!