A few points that have not been covered:
-
The amp that the audio will be tapped off from, is the output driving the speaker with reference to common ground or is the amp using some kind of bridge circuit (common in lots of audio driver ics) where neither speaker lead is referenced to ground?
-
Using a compander like the SA575 will help regulate the levels and although it has a dynamic range of 60dB or so, what is the maximum input in dBm?
-
Keep in mind that bass frequencies use a lot of power to reproduce and although one might think the amp is only being run at say 10% power (5 watts), music passages with lots of bass could produce peaks of 10 or even 20 watts of output power.
I would use an audio transformer with a primary Z of around 10K connected to the amp and a secondary Z of around 600 ohms feeding a voltage divider circuit, some clamping diodes and possibly the SA575 after that.
The transformer would not only overcome the potential problem of the amps output not being referenced to ground, but the 10K:600 ohm ratio would work in stepping down the voltage as well.
A series capacitor between the amp and one leg of the primary of the transformer will act as a high pass filter to attenuate very low bass frequencies that could cause core saturation.