Change the G.711 audio at my RTSP/RTMP-camera-streams to ACC

I have built 3 nestboxes with cameras for watching the birds during the nesting time! So, the cameras are IP-based and they have an rtsp and rtmp-livestream that can be watched in VLC or something. A person with high IT-skills has set up an little raspberry pi- server for restream them to my website (Datarhei Restreamer). It works fine but it dont support the contained G.711 audio and the result are streams without audio at the website. The audio needs to be changed from "G.711" to "AAC" or something other that are compatible to "Restreamer" to bring the sound on the website. But i tried this now since over 2 months without an solution! I tried and try it with FFMPEG on a second raspberry. I want to encode it on the second PI as live-encoder and restream the modified "new" stream so i can use it as input for the "Datarhei Restreamer"-Server. But i have severe problems with configuring the entire setup and the output. The output needs to be a "new" rtmp oder rtsp stream with the correct audio codec, the rest can be the same as original. Just the audio needs changend! Shortly, input and output needs to be the same just the audio changed into aac at the output-rtsp or rtmp-stream! Important: I dont want to pay money for expensive streaming-services at the internet - its an hobby and no buisness. Everything has to be realized with open-source solutions (Datarhei Restreamer is open source too)