No audio with Debian 9 VM
Hello everyone,

I've read all related issues and I cannot found any answer for my problem : I cannot have any audio stream for my cameras. I used Synology RTSP stream in order to get video streams, and it works perfectly.

Both streams exist in VLC and other app.

Here is my json : 
```
{
    "platform": "Camera-ffmpeg",
    "cameras": [
        {
            "name": "Camera Garage A11",
            "videoConfig": {
                "source": "-rtsp_transport tcp -re -i rtsp://xxx:yyy@192.168.1.xx:554/Sms=ID.unicast",
                "stillImageSource": "-rtsp_transport tcp -re -i rtsp://xxx:yyy@192.168.1.xx:554/Sms=UD.unicast -updatefirst",
                "maxStreams": 2,
                "maxWidth": 1920,
                "maxHeight": 1080,
                "maxFPS": 15,
                "vcodec": "h264",
                "audio": true,
                "debug": true,
                "acodec": "libopus",
                "mapvideo":"0:0",
                "mapaudio":"0:1"
            }
        }
    ]
}
```

But I have no audio, I've checked `ffmpeg -encoders` in order to verify if libopus is included and yes : 
```
 A..... libopus              libopus Opus (codec opus)
 A..... pcm_alaw             PCM A-law / G.711 A-law
```

I have read on multiple issue that we can `To use audio ffmpeg must be compiled with --enable-libfdk-aac` but i haven't found any wiki related to it.


Any help would be appreciated : is libopus enough to get audio stream ? Do I need to complile ffmpeg lib ? If so, how ?