How to organize the switching of the audio track in the stream?

Question

fmpeg

I've been struggling with this for quite some time now.
I will accept proposals for the development of a solution.
It is necessary to organize the switching of the audio track in the stream by event.
A looped list of several files is streamed to the network by udp-multicast (it doesn’t matter what stream), the audio track for this stream is taken from another source (file, another stream, audio card - it doesn’t matter) and every few cycles or every 20 minutes. throughout the entire playlist, you need to replace the audio track with the original one from the files, and switch back at the end of the playlist.
Simply, there are two streams at the input:
1) Continuously playing audio
2) Continuously playing playlist of video files
You need to switch the sound from one stream to another by a timer, or by the event of the end of the playlist.
I tried everything I could find ffmpeg / ffserver, vlc, astra, tsplay and various combinations of these solutions and their paid or free counterparts. Everywhere, either it is not described enough or it does not work at all as described.
It is very important that the broadcast goes smoothly and without interruptions, both audio and video. And in most of the proposed solutions, when playing a new file, the streamer restarts, and with it the sound of the third-party track, which should go without delay.
I will be grateful for any answers and will provide as much additional information as possible if the description is not enough. And once again I repeat, if there is no such solution:
I will accept proposals for developing a solution.