Hi, This is the place for a good old TRANSFORMER which can couple the audio while isolating the voltages.
You should have plenty of audio out of the Arduino so you can afford (probably need) some losses to match the microphone level.
You need some "Audio Transformer".. many different ones would probably work, as you can use a 2-resistor "voltage divider" (What we would call an "Audio L pad" in my ancient Broadcasting days).
Sources?? Radio shack had some small audio transformers. something with impedances like 600 ohms to 600 ohms would be good. You could also use:
- Output transformer from an OLD transistor radio
- coupling transformer from an old dial-up modem
- small "filament transformer" if desperate
Let us know what happens ...