Hi Jon
There are two types of audio signals, "Line in" and "Mic". The difference is the signal strength, line in is much stronger signal. The AVS needs the line in signal. Most microphones are using the mic signal. There are small audio-mixer that converts the mic signal to line in signal.
If you turn the volume up very high you might hear a low audio, if so the signal you have is not a line in signal, but the mic.
Hope this can help you.
Regards,
Bjørge