yeah, the pop and cheer sound is the standard “construction complete celebration.”
Every time a building is completed, the game has to re-calculate the terrain region around it (this info is used to speed up and simplify pathfinding), which is pretty processor-intensive… it may simply be that your computer is reaching its limits of how much it can do at once when it’s calculating this info. It’s especially likely if you’re building large or complex buildings – bigger buildings are likely to cross multiple regions (think of them like chunks in Minecraft – imagine building a redstone contraption that covers multiple chunks and causes all of them to be updated at the same time) and thus create a lot more work for the computer when it has to update them.
One useful trick I’ve found is to break medium to large buildings up into rooms or sections, which are built as separate buildings. There are a bunch of benefits, although the main one is that the game only has to figure out one section of the building at a time rather than having to process the whole thing – which makes it easier and faster to build the sections too. Another major benefit is that if the building is targeted by Gary’s building-crusher tentacles, they’ll only knock down one section (assuming you don’t stop them first), so you won’t e.g. lose your entire town wall in one blow!
In the meantime, to reduce the impact of your issue I’d recommend keeping an eye on the building construction, and pausing the game when it’s nearly finished. Wait for the “resource monitor” bar (the brightly coloured bar in the bottom-right corner that’s changing colours and looks a bit like a graph – because it is one lol) to go almost fully green, and then let the game run; when the bar fills up with blue pause the game again and let the computer process all that blue, and so on. The game will still probably freeze when the building is completed, but it won’t have as much other stuff on the “waiting to be figured out” list at the time, so the freeze should be shorter and there should be less chance of any calculations getting lost/stuck during that time (which is what causes the “hearthlings are idle even though there’s work to do” situation – that’s a sign that the computer is too overloaded with calculations to figure out what the hearthlings should be doing.)
Another good habit to get into is to take frequent breaks, and not leave the game running for hours at a time. The amount varies based on different computers, but the longer the game runs the more “junk” builds up in its memory, and even though the game tries to clean up junk data wherever possible there’s some stuff that it never considers safe to delete/clean up. Also, when saving and exiting the game I like to pause it first, which seems to make it save more quickly, and more importantly it loads back in more smoothly because the game can load everything up and then re-start the simulation rather than trying to load everything + keep the game running at the same time.