How to sync audio and video for streaming with ffmpeg?

Question

fmpeg

Hello, I have a task to stream video through ffmpeg, for this task I used raspberry pi zero with raspbian OS and a program that allows you to get video from the native raspivid camera, in general, I managed to stream video in this way:

sudo raspivid -n -vf -hf -ex auto -w 1024 -h 768 -fps 25 -t 0 -b 2000000 -o - | 
ffmpeg -y -f h264 -i - -vcodec copy -f flv rtmp://myip/myapp/mystream

But you need to send a stream with sound, I tried to use alsa in ffmpeg, but too much cpu is consumed and a warning alsa buffer xrun occurs. Solved the problem using arecord:

raspivid -t 0 -w 1024 -h 768 -fps 25 -vf -hf -b 1000000 -v -o temp.v & 
arecord -f cd -D plughw:0 | 
ffmpeg -i temp.v -itsoffset 13.5 -i - -c:v copy -c:a libmp3lame -b:a 64k -f flv rtmp://myip/myapp/mystream

Using the method of scientific poke, I picked up the parameter itsoffset, sound and video go synchronously. But if the Internet speed sags (3g modem), the stream disappears for a while, then the sound and video are completely out of sync and then go asynchronously. Unfortunately, I have no experience with ffmpeg and I don’t know how to fix it, I tried to solve it by choosing async and vsync parameters, it didn’t help. How can this problem be solved? Thank you.