The goal is to ensure that sudden loud noises don't become overwhelming and quieter parts sound reasonably the same.
My reason for doing this is because it's so hard to watch movies these days because of the inconsistent volume levels (2x harder at night).
Any suggestions or recommendations on specific products or solutions would be greatly appreciated! Any DIY suggestions are also welcome too.
Lots of pro audio boxes out there.
A MiniDSP box where you can implement a compressor is fun, you can also tune just about everything else with it.
https://docs.minidsp.com/product-manuals/flex/dsp-reference/...
2 - Use a PC for your video needs. Most video players support the same function (VLC, GOM player, Kodi... look for "dynamic range compression" and similar options). A 10 years old mid-tier machine will play everything including UHD, so this solution is fairly cheap. If you get a cheap IR-USB remote, you won't even have to mess with keyboard and mouse.
It's been a while since I looked into this in detail, but iirc the decoder must automatically enable dynamic range compression when it's set to two channel output. So avoid configurations where you first convert the audio to 5.1 channel PCM or analog, and then apply downmixing or virtual surround.
If you're using a surround system, most receivers have a night mode as mentioned by others already. Some have also an adaptive system that adjusts the dynamic range based on your volume setting - turn it up and you get the full dynamic range, turn it down and the dynamic range is compressed more as you set the volume lower to try to keep the dialog audible. Many receivers include a microphone for automatic settings, and after calibration know the actual output level. The Dolby Digital bitstream (and I think DTS) include a flag to indicate the overall dialog level, and based on these a receiver can do quite a good job. Some I think go only by the DD metadata, others have a compressor that can be applied to any input.
Unfortunately modern receivers have preamps that do a ton of digital decoding, input switching and other stuff, making the audio signal between the preamp and the amp impossible to get to. That leaves you with receiver features like night mode, as suggested by others.
https://www.amazon.com/rolls-SL33B-Stereo-Program-Limiter/dp...
Specifically what you want is a "stereo compressor" or "compressor/limiter"; if you want something more sophisticated than the device above, there are many 1U rack options available for ~$200 (dbx is a good choice), or used on reverb.com more like $70-$100.
I guess that you're asking for hardware because the source device is not under your control. In theory you could use any device with a line in to process the audio. However for watching movies that may be suboptimal as any significant delay would cause lipsync issues. That's probably something you need to check for in any solution.
Sorry this sounds like an ad, but I would recommend buying any consumer soundbar with good reviews (I used rtings) before I would recommend installing custom audio equipment.
But already over-compressed stuff like music will sound worse that way, so in your position I'd prefer a software fix that is limited to movies.
¹: I am not, I like dynamic audio
That said, at night I simply use headphones.
A compressor compresses the dynamic range. They reduce the sounds about a selectable threshold with variable ratios and envelopes. Usually they have various other features.
This compressor/limiter/gate/expander/de-esser has more features than you are likely to use soon (side chaining) and will almost certainly sound good enough (despite what people may comment in response) [1].
https://www.sweetwater.com/store/detail/MDX2600--behringer-c...
Good luck.
[1] If it doesn't you can spend more later if it solves your problem. The nice thing about these compressors is: they have transparent bypass, a dedicated power button, robust construction, mature design, and are cheap.
Systems like Atmos that decompose the sound track into components might help but they're likely to get used for the wrong reason -- in the last 10 years Hollywood gave up on making the vocals in movies legible, which has the positive effect that a lot of people are used to reading subtitles, which is why you can find subtitled anime [1], Italian crime dramas [2] and such in downmarket places like Tubi these days.
(Maybe it's why my acting coach who yelled at me to enunciate the same way my wife yells at horse riders to keep their heels down is here and not in LA)
[1] https://tubitv.com/series/2082/accel-world [2] https://tubitv.com/movies/571052/mafia-millionaires-subbed