How to organize audio surveillance without streaming?

Question

CCTV

There is a 4g modem in the office with a wifi router. For some reason, I can’t reach out from the outside and began to think about the implementation of viewing from anywhere and this is what happened (the idea has not been fully thought out, there will be a question at the end):
Office 20 sq., works like a store.
We have 2 IP cameras that are being repaired, so we use webcams, mine 4g, mts router, as well as N570 netbook.
Virtual server with working site on linux and admin panel on php. Motion is used to record video.
Here's how I came up with because I'm not strong in administration:
1. Every 15 seconds, we execute a php script that sends a request to the site, checking whether an image needs to be transferred or not.
2. If you need to transfer, then every 3 seconds (depending on the speed of the Internet) we send it from the camera to the server, recording in turn either in 1.jpg or 2.jpg
When we go to the admin panel to see what is happening in the office, on the page view, automatic updating of the picture begins and a flag is written to the cache for 15 seconds that the picture needs to be transferred. Thus, we can at least somehow keep track of what is happening in the office.
As for the audio, I don't know what to do. Perhaps there will be any suggestions or alternatives?