FFMPEG demux concat dropping audio at stitch points
ffmpeg -i test.mp4 -ss 0 -t 5 test-1.mp4
ffmpeg -i test.mp4 -ss 5 -t 5 test-2.mp4


This is the root of evil. You almost never can split your audio/video with very high precise. Reasons for it:


Non-integer FPS in video. You can't split non-integer number of frames, so if you use 29.997 fps, you can't get 149.985 frames in file.
Audio codecs compress audio samples in blocks of samples, for example, in 1024 samples per second. So, if you split your sound in 5 second and sound has samplerate 44100Hz, you can only get 44100*5/1024=215.332 frames, this is too not integer. Actually, here you get small drop. Some codecs always add some silent frames at end, mp3 do in that way. Only one exception I know: pcm (any kind), this "codec" can save any count of frame, with no rough.


So, in conclusion, if you want split and concatenate files without loss:


use integer count for fps
use codecs what can encode any count of samples without rough or padding.