Hah I even replied on that thread, wow 3 years ago.
OBS Classic functions like that, system volume affects Recording level
OBS Studio ignores the master level typically, but it seems it doesn't for some audio devices like your Bluetooth
In order to hear your old recordings at the correct volume you would need to be at 100%, because OBS was recording at your system level with your Bluetooth, you're listening to your recordings at 20% of 20%, instead of 100% of 20%.
It's best to avoid all this non-sense by just running at 100% and turning stuff down, it's way easier to balance audio. 100% isn't bad, it doesn't add gain, it's Unity level meaning Input is equal to Output.
Sure it's not as handy as turning down the master volume, but considering OBS ignores that you should get used to turning down everything else instead of the master.
Gain Staging 101, limit variables.
A Gain stage is any time in your chain the volume level changes. Volume knobs, inline volume controls, digital volume controls. Windows Volume is the easiest to fix, 100% gets rid of it and ensures what is playing is exactly the level it was meant to play at. 0dB = 0dB. Use the volume mixer or in application volume sliders to bring individual levels down.
Next is your volume control on an interface or an in-line cable volume control, typically analog, these change the level you hear it in your headphones/speakers. Way harder to take into account, that is why meters exist.
TL;DR
Run at 100% so what you hear is what OBS hears. Balance your audio per application so that is is comfortable for you, take into account if you have analog volume control on your headphones you might not be hearing it at unity (input = output)
It's easier for a viewer to turn you down, than it is for them to turn you up. It's easier for you to turn up music or game audio if you run them at 20% in app, than running at 100% in app and turning up master(which doesn't work anyways)